Step one: gear up
If you are at all interested in PvP as a Retribution Paladin, you must get geared up. There are no two ways about this. Unless you have the proper gear, you will simply not perform as well as similarly-geared classes and your PvP experience will be diminished greatly. Retribution is one of the most equipment-dependent specs in the game, and you will feel it in PvP. Chris has written an excellent starter guide for all aspiring Ret Pallies, which is an excellent read for learning about the perfect Retribution gear. In order to do some PvP, we'll then need to take the next step and go beyond what Chris suggested and aim much higher as far as at least one piece of equipment is concerned -- your weapon.
Paladins are, for the most part, a glorified auto-attack class. With a severely limited selection of offensive spells, a Retribution Paladin relies on white damage more than any other class in the game, augmented with Seals. This is why your weapon selection is so important. When looking for the right weapon, try to obtain the highest top-end damage available to you, preferably over 500. Needless to say, you should find a slow two-hander. It can be an axe, mace, or sword, depending on your preference. Unlike Rogues or Warriors, Paladins gain no particular benefit from a specific weapon type so choose the weapon that suits your tastes. As much as I personally love polearms, you should steer clear of them because most polearms are too fast to deal any amount of respectable burst damage and have Agility or raw Attack Power as a stat. Retribution gains the most from Strength, so weapons that have it are good choices, as well. In a nutshell, get the best weapon possible (for you) or don't even bother.
Naturally, it pays to have a bit of Resilience, so try to stack some of it, even from gear that comes from instances or PvE. Fortunately, being a plate-wearer and having some of the best defensive spells in the game already contribute towards your longevity in PvP, so starting with low Resilience won't be too much of a disadvantage. As you wage more battles and accumulate more Honor or Arena points, you'll eventually get the right balance of offensive power and Resilience. Get gear that confers a lot of Strength, specially if you have Divine Strength, which translates to more Attack Power per Strength point. For a Retribution Paladin, it's probably the single most important attribute, so if you can't find Gorilla-type gear, don't hesitate to get Warrior plate. Don't be too concerned about Spell Damage. It's not important. Well, not that important, particularly with many changes in Patch 2.4 moving away from Spell Damage and towards Attack Power.
Step two: spec right
Now that you have the gear, it's time for you to pony up some cash and visit your Paladin trainer in order to get the right spec. Because you will be judging and re-sealing a lot in PvP, you should invest five points in Benediction. Next, two points in Improved Judgement is a no-brainer while Improved Seal of the Crusader and Deflection are toss-ups. While ISotC is good DPS and team contribution, most of the time you won't be able to judge it. Having Seal of the Crusader up during an auto-swing cripples your burst capability so it's not too wise to use it when engaged. In addition, the debuff is easily dispellable, so it sees limited use in PvP. If you can get it off at the start of the fight, great. Most of the time, however, you'll be switching targets quickly and won't have time to set it up.
The third tier of the Retribution tree is the PvP-centric tier, where all four talents are a must for PvP. Unfortunately, a PvP-optimum spec for Retribution isn't the same as the spec for raid DPS as some talents are PvP-specific and the points would be better spent on other talents if you are focused on PvE or raiding. Among these PvP talents is Vindication -- improved greatly in Patch 2.3 to affect all stats rather than just Strength and Agility, although most NPCs and all raid bosses are immune to it. Pursuit of Justice (also buffed in 2.3) is indispensable because it helps ensure that the Paladin gets within range of his target, so max it out. Conviction is also important because PvP is all about burst damage and having more crit will moderately help negate Resilience. Lastly, Seal of Command rests here so pretty much you'll need to take all talents on this tier. SoC is your bread and butter Seal for PvP.
The talents on the fourth and fifth tiers are all optional. None shine particularly brightly in PvP, but whatever you do, steer clear of Improved Retribution Aura. The aura is next to useless in PvP unless you're fighting a mob of Level 1 gnomes or something. You should almost never have it on. Crusade is better for PvE, but certainly doesn't hurt to have it. Two-handed Weapon Specialization is arguably the best talent from both tiers since you need to find ways to up your white damage. Sanctity Aura and Improved Sanctity Aura are not as stellar because the former doesn't affect Crusader Strike; it increases Seal of Command damage, though, so if you take the talent, keep it on when you're on the offensive. Then there's the neither here nor there Eye for an Eye, which isn't so much a defense against spellcasters as it is a rude middle finger to them as you die. If you deal enough damage with Eye for an Eye to hurt a Mage, for example, it probably also means she hit you for a 6k Pyroblast.
On the sixth tier, we have Vengeance and Sanctified Judgement, both of which I highly recommend because 1) the high-pitched schwing! sound of Vengeance proccing and the glowy hands make me feel warm and fuzzy all over, and 2) you will ironically have a pitiful mana pool if you geared up properly, which means you'll need all the return on investment you can get. After that, you get Repentance, a pathetic 31-point talent but simply indispensable because it's one of the few ways to catch up with an opponent or interrupt spellcasting. It is also a PvP-centric talent because it only works on humanoids and lasts for a whopping, damage-breakable 6 seconds. Beside Repentance is Sanctified Seals, which was reworked in 2.3 to be the essential Retribution PvP talent. The last thing you want in PvP is some punk Shaman Purging away your Seals and leaving you swinging an impotent wiffle bat. Divine Purpose fits some damage-mitigation builds but is entirely optional.
On the eighth tier, there is Fanaticism, which is an awesome talent for PvE, but also helps in PvP. Some damage-mitigation-oriented PvP Retribution builds skip Fanaticism in order to go up the Protection tree, getting Improved Righteous Fury to complement Divine Purpose. Such builds often also take Improved Hammer of Justice which, when coupled with a PvP 2-set bonus, allows the Paladin to stun every 35 seconds. Finally, there's Crusader Strike, which is the only controllable attack a Paladin can make aside from Judgements. Crusader Strike greatly reduces randomness and reliance on luck and gives a Retribution Paladin some semblance of controlled PvP damage.
Aside from some key PvP talents, feel free to throw points into talents that suit your playing style. While some go up Protection, others go up Holy and take Spiritual Focus for some self-healing love while picking up Divine Strength along the way. Some have two points to spare for Unyielding Faith, which helps against those annoying Warlocks. If you have don't have enough +hit (you'll need about 5% for PvP), you can go for Precision, but will be wasting five points on either Improved Devotion Aura or, um, Redoubt. Once you've tailored your Retribution spec -- I personally prefer pouring points into talents for high burst -- it's time to get down to business.
Step three: smash face
Find your nearest Battleground or Arena Battlemaster and queue up. If it's the latter, I hope you didn't forget to bring a team. You can also go out into the world and unleash your power upon unsuspecting questers and gatherers. No, I don't recommend it, but if you want to play the role of a twisted Blood Knight to the hilt, griefing will be right up your alley. Of course, I personally think the Battlegrounds are more fun, so let's pretend we talked to Yula the Fair or, if you're Alliance, probably some blonde gym rat in a loincloth and got sent into battle. This is where the real fun starts.
The first thing you have to keep in mind when PvPing as a Retribution Paladin is that you have no ranged attacks. None. Nada. Zip. Zilch. Don't even mention Hammer of GG because it's an extremely conditional spell that takes a half second to cast. This means that it's a half second slower than a Warrior's Execute or a Shaman's Frost Shock, so killing blows with Hammer of Wrath must be exceedingly well-timed. Either that or you can be fighting solo, which is also a lot of fun. Anyway, the point is that as a Paladin, the only way you'll be able to kill anything is by being close enough to mash their helmet in so do everything you can to close the gap. For the most part, this means a lot of running around. Paladins are ironically a melee class with exactly one melee attack spell, Crusader Strike -- which is a 41-point talent.
If there's any seal you'll want to judge on an opponent before the fight starts, it's not Seal of the Crusader but Seal of Justice. Because it caps your opponents' movement speed at 100%, it helps ensure that you will maintain at least the same pace as your opponent. If you have Pursuit of Justice -- which you should since I already said you should pick up the talent seven paragraphs ago -- this will reduce your chances of getting kited and help compensate for lag which will often tell you that your target is "too far away" even though your monitor shows that you're right on top of them and close enough to smell their cheap cologne. Judging Seal of Justice on mounted opponents in Alterac Valley is probably one of the more fun things to do because you'll get a lot of free swings while your enemy tries to get away not understanding how you're running as fast as their mount. Of course, you and I both know it's actually the other way around, since their mount's slowed down to your pace, but they can figure that out while checking out their Spirit Healer's pedicure.
Don't save your stun. There is no special, magical moment for you to use your Hammer of Justice. If it's up, use it. Paladins often make the mistake of trying to use it as a spell interrupt. Don't. If it's up, use it and hope that you survive long enough to either kill your opponent or use it again. The sooner you use it, the sooner it will become available for you to use another time. While it works as a spell interrupt, it is also highly resistable by many classes (and even races... those Orcs are total pigs about it) so it isn't very reliable anyway. You should always have Seal of Command up, so a macro that uses Hammer of Justice and Judgement together helps. It looks a little like this:
/cast Hammer of Justice
Since Judgement doesn't share a global cooldown with your other abilities, it can be cast in the same breath as a replacement Seal. You can have it as a macro that looks like this:
/cast Seal of Command
This ensures that you will always have Seal of Command up even after Judgement. Of course, you'll encounter the occasional "Spell isn't ready yet." message, but generally the macro will keep your Seal up most of the time specially since you will have to Judge often. About 8 seconds often, to be precise. You need to keep Judging you opponent because it's one of the few sources of damage that you have. You do not have sources of periodic damage (no, Seal of Vengeance doesn't count), no ranged attacks, and no true form of crowd control, so keep attacking, spamming Crusader Strike, and Judging. The idea is to deal as much damage as you can in as short a time as possible -- before someone pulls off a heal, at which point your should roll your eyes up and groan loudly.
You might wonder, what about Consecration? Well, consecration is a Shockadin's tool. Unless you're old school and stacked tons of +spell damage, Consecration won't be worth much in PvP. Use it only against Snake Traps or spam Rank 1 to flush out Rogues or maybe Night Elves secretly having a sandwich. Otherwise, Consecration is a great way to quickly deplete your already low mana pool and should be avoided. Crusader Strike and Seal of Command are your staples in PvP, and you could probably do well with a short macro that attacks and casts Crusader Strike at the same time:
/cast Crusader Strike
If you have Seal of Command on, all three attacks can hit or even crit simultaneously. There are few things sweeter than surprising an opponent with an instant burst of 7k damage from crit white attack, crit Crusader Strike, and crit Seal of Command. You can immediately follow this up with a stun and Judgement. While on the offensive, don't hesitate to use Repentance to keep your opponent incapacitated and continue the assault. With a little luck, Retribution burst can sometimes take down an opponent even before they've made a move.
Pick your targets. It's a fact that Retribution has limited offensive options, and while burst damage potential can be rather good, it is often not enough to take down a decently geared healer. This is the rock to a Retribution Paladin's scissors. Because you can only burst in three different intervals -- X seconds, 6 seconds, and 8 seconds, where X is your auto-attack speed -- healers have plenty of time to pull off a heal. Paladins can't remove shields or other buffs, and have exactly two (three if you count Arcane Torrent) spell interrupts on one minute cooldowns, which severely gimps damage-dealing against healer classes. My advice? Don't even try. Part of doing well in PvP is knowing where you're beat. If you take a whack at a healer class and see that their HoTs tick for more than you can dish out, it's time to find some help. If you luckily find a healer type that's soft, squishy, and hasn't quite caught on the latest Resilience fashion trends, then by all means instagib them. Otherwise, it can be severely frustrating and ultimately futile to wait on ability cooldowns and a slow weapon to slow down a healer's heals.
With other classes, Paladins stand a fair chance, specially with Divine Shield used as an offensive weapon rather than a defensive fallback. Don't take the risk. It's very easy to go down from 20% to 0% health before you can press your key binding. Don't hesitate to charge at an opponent with Divine Shield on... this catches most enemies by surprise because most of them expect it to be used when you're low. Deal as much damage as you can while immune... and while this means slower auto-attacks, it also means you can move around freely. Considering that you always need opponents close, your movement is at a premium. Being able to burst through with high amounts of damage while being invulnerable is a great advantage in PvP. Use Blessing of Protection to remove Physical debuffs such as Hamstring or Blind. Remember that while you can't attack through BoP, you can cast Judgements and Hammer of Wrath, so your assault can continue even if you can't swing your weapon.
Finally, remember that you're a healer, too. Even though your healing might be gimped, you can create a macro to equip a healing weapon, shield, and libram together with your Holy Light or Flash of Light spell. Heal yourself mostly between battles, simply because it's significantly mana-inefficient to heal rather than press on with the attack. Against Rogues and Warriors (and even other Retribution Paladins), use BoP to catch up on life or break their pace. Throw up a Rank 1 Consecrate so they won't be able to bandage while you heal yourself. You also have Lay on Hands, which is your get out of jail free card, provided you don't have some sort of Mortal Strike debuff on you. Don't hesitate to use it. Sometimes, the extra life will allow you to take down more opponents. If you pick your targets right, you'll find that a Paladin's natural resilience will allow you to take down multiple opponents before being overcome. Remember that most of your damage is Physical, meaning you will deal considerably less damage against plate-wearers and even some mail, specially mail and shield. Don't be surprised to see your normal hits drop from your usual 1k to half that number against targets with high Physical mitigation. Again, it's important to pick your targets well, choosing those with low mitigation before moving to the next opponent.
The Paladin is a challenging class to play. Once you've managed to wade through 70 levels of auto-attacks, you might eventually find that it can also be very rewarding. Despite the ridicule and spite Retribution Paladins receive, there's no question that it can be major fun in PvP. Besides, you can use it to your advantage as you can split opponents' skulls in two while they're rolling over in laughter at the sight of you. Retribution, as some people say, is rather sweet.