Holy paladin resources
I like to use clcbpt to manage my holy paladin buffs, along with a few other addons and macros that make my life easier. Holy paladins use the new holy power system to build up holy power points and then release them via Word of Glory or Light of Dawn. We're a caster class, but we still wear plate armor, so we are fairly resilient to incoming damage and can usually survive more punishment than the other healers.
Wait until level 30
If you want to level a holy paladin, you're going to have to kill all of your mobs the slow way. While every paladin is identical until level 10, once you're able to choose your talent trees, you can choose to focus on the holy tree. Holy Shock, the tree's signature spell, actually does reasonable damage, but not nearly as much as the other two trees are capable of. Retribution and protection paladins gain access to valuable DPS spells and talents early on, while holy paladins are left in the dust.
Since you won't be able to pick up Denounce until level 31, you won't even be able to use Exorcism as your primary damage-dealer until then. There's really no advantage to leveling as a holy paladin until level 30. You'll be attacking mobs with your weapon since you don't have a valid nuke spell to cast, and you can easily heal your group through any dungeon while using a retribution or protection spec anyway.
Denounce and dual talent specializations
Once your paladin is level 30, the holy tree instantly becomes viable. You gain access to Denounce, which lets you chain-cast Exorcism all day long. More importantly, you can unlock your dual talent specialization, allowing you to switch between two different specs at any time. I suggest using a protection or retribution build for handling your normal questing and grinding, and then setting your secondary spec to a holy build to heal in dungeons.
Using the random Dngeon Finder, you can quickly and easily find groups for dungeons near your level. The Dungeon Finder is a great way to level up, as you can complete lucrative dungeon quests and obtain awesome gear from the bosses. Holy paladins are plenty potent enough to heal any of the leveling dungeons, and you can practice your healing skills in a relatively safe environment before taking your game to the big leagues. If you pick up Denounce, you can also contribute a bit of DPS when everyone is at full health.
Managing a low-level healer
The most important aspect of low-level healing is to manage your casting time wisely. As you're leveling up, you're still missing many of the cooldowns and advanced spells of a fully developed paladin. You'll be spending most of your time using our cheap heals, like Holy Light, Holy Shock, and Word of Glory. These three spells can handle nearly any situation you'll run into. Flash of Light is best reserved for emergency situations, and even Divine Light won't see much use outside of boss fights when you receive it at level 62. Holy paladins have always had simple healing spells augmented by a robust toolbox of secondary spells, but dungeons are typically so basic that our utility abilities aren't necessary.
Holy paladins leveling through the Dungeon Finder should also be cautious of their mana pool, as many tanks won't always stop to let you drink, and you never know when your group might accidentally pull an extra group. We can use our Seal of Insight and Judgement combo to regenerate mana during encounters, and Divine Plea can easily be mixed with a drink of water to fill your mana bar between pulls. Your mana pool should be fine if you stick to the basic heals, saving Flash of Light and Divine Light for emergency situations.
Your final goal will be to manage your buffs to ensure you're operating at peak efficiency. You need to keep Seal of Insight active at all times, as Judging to restore mana is important. When you use Judgement, you'll get a haste buff from Judgements of the Pure, so make sure you're judging at least every minute, if not more often. Beacon of Light becomes a valuable addition once you pick it up, as it makes handling AOE damage much simpler. You'll want to keep Beacon of Light active on someone at all times. The addons I mentioned earlier can make all of these tasks trivial.
Choosing holy paladin gear
One of the worst parts of leveling a holy paladin is the severe lack of plate gear with intellect on it. Once you pick up Plate Specialization, you'll want to be wearing all plate armor if you possibly can. Intellect is our primary stat, and every piece of gear we wear should have intellect on it somewhere. Since holy paladins aren't considered to be offensive casters, our gear will never have hit rating on it. Spirit, haste, and critical strike rating are the only secondary stats you'll find on our gear, and they're all similarly balanced. I find that crit and spirit are actually incredibly potent for the low-level healer, while haste tends to not make a very big difference. Either way, as long as you're wearing plate armor with intellect on it, you're good to go.
The other negative aspect to holy paladin leveling gear is that we can't use staves or daggers, which make up the vast majority of caster weapons we'll see while leveling. I recommend snagging a Devout Aurastone Hammer from the heirloom vendor if you have a high-level character already, as it resolves the weapon issue completely. Note that the holy paladin heirloom gear will look like mail armor at lower levels but will convert to plate armor once you learn the ability to wear plate armor.
The Light and How to Swing It: Holy helps holy paladins become the powerful healers we're destined to be. Find out just how masterful mastery healing can be, gear up with the latest gear, and learn how to PVP as a holy paladin.