The Priest class are a fairly decent class for leveling in WoW, although certainly not the best! I have only tested one suitable leveling build for the priest which was primarily a shadow build. The Shadow Priest build is simple to apply as nearly all the available talent points will be spent in the Shadow talent tree.
The key here is also to move down the tree quickly scooping up the better abilities that will serve you better dependant on which stage of the game you're at.
To kick-start the Shadow build I would take Improved Shadow Word Pain as this will improve the Priests damage dealt. Then I would grab the Darkness ability and skip Veiled Shadows for now.
In tier 2 I would take all abilities and assign all talent points to these abilities except for the Twisted Fate ability which can be skipped for now and taken a little later. Now you want to move down the talent tree quickly to unlock the Vampiric Embrace ability.
To move down the tree I'd take the Harnessed Shadows ability and apply 2 talent points here, then I'd assign 1 talent point to Shadowform. Now you'll be able to move down the talent tree again where you can obtain Vampiric Embrace which is a great ability. The Silence ability is nice to and I would spend a talent point here next.
Mind Melt and Machochism are next on the list for the Shadow build and 2 talent points can be assigned to each of these talents. At this stage your Priest should be a nice build with decent damage abilities and a strong resistance against attackers.
To move further down the talent tree you'll want to go back to both the Twisted and Faith and Veiled Shadows ability that were skipped earlier and then take Vampiric Touch. Directly from there you'll now be able to take both the Sin and Punishment and Shadowy Apparition abilities assigning 2 talent points to each ability.
Now you can take the prize ability Dispersion and your build is almost complete. To finish up you'll want to assign the last available talent slot in Shadowy apparition and 2 points to Paralysis, Pain and Suffering and 1 talent point in psychic horror.
That's the Shadow Priest talent build complete, as you can see most talent points have been assigned to this talent tree making the build easy and a reliable build for leveling.
To finish the build I would spend my last 4 talent points in the Discipline tree on whichever talents you find best suitable. You'll find that the Priest class aren't the easiest class to level in WoW but they are a great deal of fun and fit into groups well due to their unique abilities.
The build mentioned above should serve you well in both solo quests and grouping in Dungeons.
