Holy Priest:

Yay heals and Raid buffs! Who could ask for more?
What Stats are important for you?

- Bonus Healing: This stat will increase the healing from your healing spells.

- INT: Increases your mana pool and indirectly effects your mana regen through your spirit.
You should not need to gem for int as it will come on most gear, but if you have a
tiny mana pool you're useless.

- Spirit: This is one of your most important stats because you have so many talent points
that will increase your healing and mana regeneration.

- Mana per 5: You don't need to stack this stat straight, but it often comes on cloth gear. Gem for spirit instead of Mp5

- Stamina: Will already come on most of your gear.

- Haste: Once you have sufficient gear with lots of +heals and regen, you may begin building your haste set.

Enchants:

Go for Plus heal and spirit on all of your enchants. =) Balance them based on your gear!

Buffs:

Keep up your raid buffs and always carry candles. If a pally tries to give you blessing of wisdom, kill him and ask a different one for Blessing of Kings.
Why Kings? Kings increases your stats, which INCLUDES SPIRIT!!!! More Spirit means more mana regen for you. =) It also gives you a
little extra stam so that it is far better for you than Wisdom. ^^ Train your pally friends! If you can get both, take both... otherwise kings pwns face.

Specing:

Most of your talent points will go into the holy and discipline trees. Your holy tree
has talents to increase your healing effeciency while discipline is focused on mana regen.

I recommend taking Improved Fort in the Disc tree as well as Improved Power Word: Shield and Meditation.

When looking to your holy tree, what you need to avoid: Talents that reduce stun and fear times, those are for PvP.
Lightwell should also be avoided because it sacrafices too much healing and most groups will ignore it entirely.

While Healing:

You have 5 major heals:
Greater Heal- this is a slooowwwww spell to cast, but it heals for a great deal. 3.1 - 4.1/mana base making it mana effecient, but taking a long time to cast.

Flash Heal - This is your fastest casting spell and is used to heal someone quickly but for very little. This restores around 2.5hp/mana base, so it costs.

Prayer of Healing - Heals YOUR party memebers within range at a mid level and should be used to recover your party quickly.
This is important because healers are generally responsible for their own group unless otherwise told.

Renew - This is an instant cast HoT (heal over time). You should keep this ticking on your tank at all times.
It's also effecient if you know that there is AoE raid damage coming. Putting a tick on each player
in or outside of your group can save you on mana and help out the entire raid. This is a 2.5 - 3.4/mana base depending on your spec.

Circle of Healing - This takes a nice amount of mana, but is instant cast. If there is AoE damage and you need to
Top off an entire group quickly this spell is ideal. It will also own healing meters if used correctly.

Added (2009-07-16, 3:49 Pm)
---------------------------------------------
Now that you've been introduced to the proper usage of your spells, let's talk about Healing Priority and managing your mana!

What is healing priority?
Sometimes as a healer you have to make a choice. You get to play God. You must be able to decide on the turn of a dime
who will live and who will die.

When it comes down to it the following is most important: The Tank and then the Healer and lastly the DPS.

If it comes down to letting a DPS die, versus letting another healer die. Heal the healer. If it is between yourself and another healer,
you've read this guide so you know to heal yourself! After you comes the highest DPS on the encounter. If the fight requires high single target
save the player that puts out the highest single target DPS, if it requires AoE, save the player that puts out the best AoE.
You will often be assigned to heal your group so you are responsible for their deaths, but you must remember that if certain people die, the raid will fail
anyway. The most important ppl may not be in your group!

How much mana should you use to save someone?

This depends on their priority. If you can get someone from 30% health too 100%, but it costs most of your mana it's useless. Throw off a rank 2 or 3 greater heal to put them at 60% instead (if you can spare the time to heal)!

Defensive Healing:

Please remember that you can cleanse magic and dispel diseases! These things should be on your bars and key-bound! When it comes down to it, cleansing debuffs that cause a great deal of damage will save mana and healing in the long run and make the raid more successful. If there is something you cannot remove, such as a curse, ask the druids or mages in your raid to do it.

Fear ward: It's great in cases where fear is a problem and you have no shaman for tremor totem. Keep it on your tank or yourself to ensure a smooth raid.

Offensive Healing:

Some mobs will have magical buffs on them that can deal a large amount of damage unless dispelled. Your dispel will also work like a purge and can be helpful!

Things to Remember

* Stick to healing priority no matter how much some players complain, explain what you are doing and why and ask them to manage their aggro better so that they will not need heals.
* You can not keep anyone one up if you are dead, ensure you watch your own health bar too.
* Never shield a tank since it will cause them to drop aggro!
* When healing a large number of people either use your renew or a group heal.
* It is critical to keep buffs up on everyone in the group.
* Remember you can fear MOBs off of you if you are really stuck, just be cautious of your position as if you do this too close to other MOBS it may aggro them.
* Remember all the heals you have and use them appropriately.
* You are a Healer, a critical member of a group. You don't need to put up with crap, if the group is bad or bossy, tell them to suck it.

Once again check out: http://www.wowwiki.com/Priest_PvE_guide
It is helpful to priests and non priests alike!