I play two priests, one shadow and one is going down the discipline path. Since the 2.3 patch leveling has become insanely easy because of the increased xp per kill/quest.
I play mostly solo, so this is what I do. I line up all my quests so I am not wasting time running around willy nilly. Get a nice cartographer mod w/coordinates. I also use Lightheaded for notes on quests. Makes it quicker to get multiple quests done in a row = more xp = faster lvling.
Second, pre shadowform, I used the following spell rotation. Smite Smite, SW:P shield, mind blast, wand to finish them off.
I carry a set of healing gear with me, but don't worry about that too much, just keep plenty of mana pots, bandages, etc in your bags at all times. Once you get to Outland, all your gear will get much better.
Run instances for your level, get all the quests lined up before you go. You get good gear/quest rewards plus a lot of XP.
I hope I managed to answer some of your question.