I'm not really going to tackle the spells in order of level, but more in order they need to be talked about. We'll do a couple what spells to avoid and then we'll work our way through the ones you do want.
When tanking, you're going to want to make a point of using only instant cast spells. If any spells require you to sit there and magically wave your hands for a couple seconds, then you should avoid using it while things are beating on you. This means Holy Light and Flash of Light are out. Exorcism is out except for pulling new mobs. And while we're on the topic of things that are out, Redemption isn't much of an option to any paladin in combat, but that's for a completely different reason.
There are also two other spells that while they can have some great uses while tanking, generally aren't something you want to cast on yourself and leave on. Those are Hand of Protection and Divine Shield. There lesser sibling, Divine Protection that you get at level 6 is perfectly fine though and we'll get to it in a second. The reason you should avoid using HoP and DS is that while they're active, your threat is zero. Anything attacking you will suddenly lose interest and go find another person to play with and your healers really don't like it when that happens. If you cancel either of those, your threat will be back to where it was beforehand. This can be handy if something jumped out of nowhere to attack your healer or one of the dps, you can cast Hand of Protection on them which will drop their aggro to nothing allowing you to pick up the target easier.
The first buff you'll want to cast on yourself as a tank is Righteous Fury. This is the key spell to tanking as a paladin. Without this buff, you'll have a really, really hard time keeping your threat high enough to do anything. This spell boosts the threat of any holy based spell by 80%. That means everytime you hit a judgement, you're getting almost double the threat out of it.
This used to be one of those buffs that you'd have to re-apply every 30 minutes. That means if you weren't paying attention, it could just drop off in the middle of a fight. Now, however, it's a permanent buff much like your auras. Once you put it on, you have to manually remove it (or die or run Magister's Terrace, because it can be dispelled and there's a fight in there where you can have it stripped off of you... we hate that fight). I actually have something setup in the Power Auras addon that will put a big flashing red shield in the middle of my screen when I don't have this buff cast so that I never forget it and will always notice if it gets dispelled.
Now that you've got Righteous Fury up, you'll need to pick your blessing and before level 30, you'll be using Blessing of Kings which you get at level 16. This will increase all of your stats by 10% which means more stamina for health and more strength for threat and block.
Past level 30, Blessing of Sanctuary (an ability you get via talents and not from a skill trainer) is going to be the defacto blessing you'll have up. It also increases your stamina by 10% for added health as well as your strength by 10% to up your threat. However, instead of giving you 10% in the rest of your stats, it decreases incoming damage by 3%, which is something I'm sure anyone can appreciate. Lastly, it also gives you mana back whenever you dodge, parry, or block an attack. Being that this will be the only way you have to get mana back for quite some time other than Judgement and Seal of Wisdom, it's a very important blessing to keep up.
Lastly, for getting our long term buffs ready, you're going to need to pick a seal. For most of your tanking career while leveling up, you'll be using Seal of Righteousness. It's a solid little seal that will serve you quite well until your mid-sixties. Some people still like using it past that and depending on a couple of different variables, it's actually pretty good. I, personally, like it for fights where I want to avoid any damage over time effects related to some of the other seals due to strange threat issues with the fight (Hydross, Twin Emperors, etc).
At level 66, you'll be able to go learn a new seal from your trainers. If you're an Alliance paladin, you'll learn Seal of Vengeance. If you're a Horde paladin, you'll learn Seal of Corruption. The important thing to know about these two seals is that they're the exact same. No, you're not getting a lesser version than the other side. Any talent, bonus, or glyph that affects one always affects the other. They're just named differently.
The way this Seal of Corruption/Vengeance works is that every time you hit your opponent, it will put a damage over time effect on them named either Blood Corruption or Holy Vengeance depending on your seal. Once the stack has built all the way up to a maximum of five, you'll do an extra 33% damage with every hit. Also, the judgement damage related to this seal builds up with each extra charge on your debuff stack.
One seal to keep on the back burner for special situations is Seal of Justice. It is usually dragged out for use against casters as it has a chance to stun your opponent with each hit you make and can interrupt their casts. While this feature isn't useful in every scenario, it is a good one to know about if you ever have a need for it. However, due to it's lack of threat, you'll almost always want to use something else.
Speaking of judgements, your choice in which judgement to use really depend on what you happen to need. If you need mana, go with Judgement of Wisdom. If you're fine for mana, do Judgement of Light. If this dungeon has a lot of guys that try to run away and grab their buddies, then hit Judgement of Justice (it's also good in PvP).
Your key tanking cooldown is going to be Divine Protection. It reduces all incoming damage by 50%. You can use this when preparing for a large incoming hit or spell from the a creature or if you know that at a certain percentage, the boss just goes crazy and starts hitting twice as hard as he used to. One of the downsides of this skill is that it causes a debuff known as Forbearance. This means that you can't have any other spell that also causes this same debuff cast on you for a couple minutes. Other Forbearance spells include Lay on Hands, Divine Shield, and Hand of Protection.
A lot of the time, tanks will announce to healers when they use skills like Divine Protection, due to the fact it usually gives healers a small break from tank healing while it is up and they can take care of the rest of the party. Because the skill is very similar to the warrior ability Shield Wall, people often announce Divine Protection by the warrior skill name instead. This gives the healers one phrase to watch for instead of two. Traditions on your server or in your guild may vary though.
Lay on Hands used to be another key tanking cooldown that was sometimes used in conjunction with Divine Protection, but it now causes Forbearance when cast on yourself. However, if you're low on health and need a big heal, you can still use it, but that locks out your other main tanking cooldown. This isn't to say the skill is without merit. One of the things I often use it for is quickly healing healers who are too busy paying attention to everyone's health bar except their own. If it looks like a healer is taking too much damage from just area of effect spells and about to die, I'll toss a Lay on Hands on them so they can keep me up and running.