If you plan on leveling a Priest, for PvP, healing, or whatever reason, shadow is definitely the way to go. Even as shadow, you can heal early instances really easily regardless of what spec you go with. But if you're going to be soloing you'll want to be shadow.

Shadow priests are one of the most mana efficient killing machines to level. Simply put, you'll spend far more time taking out mobs than spending that time sitting down to drink in order to replenish your mana or health since you will have access to talents and skills that will do this for you.

The play style is very similar to affliction warlocks, without the pet. However, shadow priests have their own unique twist to the dot and drain tactics the warlock has mastered.

Unfortunately the shadow build doesn't really shine until you reach level 40 and have access to Shadowform. It can also be a fairly tough grind to hit level 20, but once you get Mindflay (at 20) and Devouring Plague (also at 20) things will start dropping quicker.

At first, your spell rotation will look a bit like:

Shadow Word: Pain, Mindblast, Smite, Smite, Wand

Until 20 when you get Mindflay:

Shadow Word: Pain, Devouring Plague, Mindblast, Mindflay, Wand

To the point where at 50 you'll have Vampiric Touch which should always be your opener.

As for stats, you really should be focusing on spirit and spell damage. Of course, not a whole lot of gear has spell damage on it early on, so look for items that have both intellect and spirit on it. Anything "...of the owl" is perfect for a shadow priest as it contains both.