So this is the basics for leveling a priest with the help of Lazy-Macros, you can head to www.lazymacros.com for more macros for each class. I have used basic macros from that site and edited them to make them usable at lower level ranges for maximum leveling speed.
B.O.A (Bind On Account) Gear is a the best option for leveling but not essential. And try get into a level 25 guild asap they add 30% exp boost.
Level Range: 1-5
So for the beginning levels you dont have much in the way of spells so spam Smite and when you get Shadow Word: Pain spam that on multiple mobs and finish them off with Smite. At level 5 you will get Power Word: Shield.Level Range: 5 - 10
Here is where it gets fun Copy and past this to a new macro
#showtooltip Power Word: Shield
/castsequence reset=0.3 0,Power Word: Shield
/castsequence reset=target Shadow Word: Pain,null
/castsequence reset=5 Smite, Smite, Smite
/startattack
/script UIErrorsFrame:Clear()
/console Sound_EnableSFX 1
Just spam that button and you will do a rotation to kill mobs.