WoW Priest Leveling Guide - Use a Leveling Addon
Most people looking for a WoW Priest Leveling Guide are generally looking for ways they can level a priest as fast as possible.
This is because most believe this class is the hardest to level in the game. The reality is a good WoW leveling addon makes getting any toon to the level cap relatively easy.
As far as what talent build to play for your new priest, I recommend trying out all 3 as you level so you can get a feel for what you will want to do once at the level cap.
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A good starting point would be the Shadow tree because you are really going to be limited for healing at lower levels.
Middle levels are usually pretty good for checking out the Holy tree because you will have more survivability with higher level healing spells and you will probably want to run more dungeons. You can get into groups pretty fast as a healer and the 40-60 range is very easy to heal 5-man groups.
I always recommend any new WoW priest try out the Discipline tree once you get closer to the level cap because it is the most fun talent build in my opinion. This spec works well for dungeons, tank healing in 10 or 25 man raid groups and is simply a blast in PVP.
Of course all of the talent builds for this class are extremely powerful and fun at the level cap. The Shadow build can pull some pretty sick damage for players who understand how to play the class.