I'm going to assume that since you're playing a priest you want to heal some and damage (DPS) some. After all at 15 you get random dungeons and healers get in easy, right?
So look over the talents and then chose discipline. Yes Discipline. Your father's brother's nephew's cousin's former roommate may have had a reason for saying holy or shadow was better but they're wrong, and I will explain why.
Lets start with Shadow. This should be a no brainer but I keep running into 'Just spec shadow and you will heal fine broah'. This is wrong, and always will be unless you so overgear the place you dont need a spec any way. Heals are expensive, why both Discipline and Holy get bonus mana regen - Shadow just helps you spend it faster. Its specialization abilities, Shadow power and Shadow Orbs are wasted. Yes you will kill things fast soloing but that means you only a damage dealer with a side of light inefficient heals.
So what about holy? At first it looks promising, with a short single target stun that causes damage being useful in healing (prevents damage from coming in for a moment) and soloing. But Holy Word: Chastise has a 25 second cooldown making it of limited use leveling when you are bouncing from target to target every few seconds and often the same when grouping. When a flash heal can fully heal your target having it 'hit' 15% harder isnt the most useful thing in the world. So for grouping Holy plays as OK and for leveling...well it is still Holy.
Discipline gets Penance a channeled spell that heals a target signifigantly for cheap or damage a foe for less than it heals and is still cheap. If you hide behind your bubble you can cast it and not worry about having the channel interrupted by unfriendlies. After three talent points it gets +6% to healing further eroding holy's uniqueness to healing 9% better. The 15% increase to intellect meanwhile means more mana, more spellpower, and more crit, so you not only heal you kill faster.
Discipline isnt as good at killing as Shadow, but only by a hair. It is an equal or better at healing than Holy. at least until level 30 it is the spec of choice to level with. My suggestion is to grab a dual spec of shadow at 30, if you reeeeally want to go Holy swap from Discipline then.
At 10, 11, and every odd level after you get +1 talent.
10, 11, 13 = Twin Disciplines +3 - +6% healing and damage
15, 17 = Improved Power Word: Shield +2 - +10% shielf damage absorbed
19, 21 = Evangelism +2 - Each smite makes all smites, Penance, Holy Fire, hit harder and cost less mana.
23 = Archangel +1 - Activate to restore some mana by removing your Evangelism buffs and increase your healing for a time. Good to smite a bit in a group then pop big heals when needed.
25, 27 = Mental agility if your having mana issues or Soul Warding to be able t cast your shield on more people.
29 = Atonement +1 - Half of your smite damage heals the lowest hp person in the group 25% if its you. So soloing you will always heal yourself 25% of yoru smite dps.