Raids are almost exclusively max level only. There are still level 60 and 70 raid instances but they're very rarely run on a serious basis. I don't know of an end all, be all guide to holy priests. You just gotta dig around the wikis and forums.
There's some good strategy at
Holy Priest - WoWWiki - Your guide to the World of Warcraft
That page covers alot. It's not hard to cast heal, you just want to keep practicing how to conserve mana, when to renew, aoe heal, flash heal, or greater heal. One thing to remember is you have more than 1 spell. You have aoe, instants, heal over times, dispel quick and mana inefficient or slow and mana efficient heals. Stopcasting is a strategy you'll need to learn for raids. That's covered on the wowwiki page.
I hope it helps.