Elizabeth wrote a great post about the initial levels of Priesting. I'm assuming that you've decided to actually commit to rolling and leveling a Priest. If you're not sure about the class yet, you may want to read up on it some more in terms of what they can and can't do. I'll discuss talents below the level by level breakdown.
Level by level break down
Level 6: You get access to Power Word: Shield, and a buffed version of Smite. What Power Word: Shield does is it places a protective shield around you which absorbs damage dealt to you by external forces (mobs and spells but it will not protect you against fall damage). With the shield active, your spells won't be interrupted. At the same time, you'll have a debuff up known as Weakened Soul which effectively means that you can't shield yourself for a period of time. Your spell cast order will change slightly at this point. Instead of Smite, Smite, Smite it will instead now be Power Word: Shield, Smite, Smite, Smite.
Level 8: The first heal over time spell in the game is now available to you. Learn to embrace Renew. You will be using often throughout the game on yourself and on others. Instead of a direct heal, it functions as a spell that periodically heals a target. More importantly, it is an instant cast spell allowing you to drop it on someone who took some damage but isn't expected to take any further damage. A common tactic with Renew is if a friendly target took a hit but isn't the focus of the mob is for you to drop a quick renew on him before switching back to healing the main player who holds attention on the enemy.
More importantly, you get another primary using spell called Fade. It temporarily reduces your aggro for a small period of time. If your healers are causing you to pull threat, use this to buy yourself some time for other players to jump ahead of you. Note that Fade is going to largely be useless if you're not in a group with other players.
Level 10: Level 10 is the first big hoo-rah level for most Priests. Your arsenal starts opening up. Spells like Mind Blast and the second rank of Shadow Word: Pain are now at your disposal. Not only that, now you've earned god mode powers with the ability to cast Resurrection among other players. This is also the last level you'll get to train and use Lesser Heal before graduating to stronger heals in the later levels.
To further cement this landmark occasion, you are granted access to your first set of Priestly racials:
Again, spell cast is going to change slightly at this point. I still open with Power Word: Shield and Smite, but my third spell will be a Mind Blast instead followed by repeated Smites. If you're facing chunkier mobs, feel free to throw a DoT on them such as Starshards (if you're a Night Elf) or a Shadow Word: Pain. By chunky mobs, I mean mobs that are a good number of levels higher than you or select bosses.
Level 12: Sadly, not a lot of cool toys at level 14. Your longevity does increase with an upgraded Power Word: Fortitude and Power Word: Shield. A new spell called Inner Fire is introduced which will help increase your survivability by upping your armor count. Remember to keep this spell active at all times.
Level 14: Your highlight spell you gain at level 14 is Psychic Scream. This Fear type spell is a doubled edged sword. Know when it's appropriate to use and understand when it is too dangerous to be considered. Psychic Scream is the Priest's ultimate red button. Think of it as an in-case-of-emergency-break-open-glass. Mobs within melee range of your Priest will become frightened and run around the area potentially attracting the attention of multiple mobs. Ergo, if things are getting frantic, it's best to try to pull the enemy (or enemies) from other mobs before lighting up the Psychic Scream.
When you do blow Psychic Scream, open up with heals on yourself and watch for the effect to wear off. Squeeze a Power Word: Shield upon yourself and you might be able to land a slightly improved Smite or two on melee mobs before they get within striking distance. Again, I reiterate, it is a last ditch spell.
Don't forget to train your next rank of Renew. Cure Disease will also come in handy for the rare Diseases you might encounter at this point in the game. Keep it handy on the bar.
Level 16: Nothing major at level 16 other than a brand new healing spell aptly named... Heal! Heal packs more of a punch than it's counterpart Lesser Heal. Depending on how you talent at this point, Heal might not take the full 3 seconds to cast. If you don't have the talent reduction, keep Lesser Heal on the bar as it's still slightly faster than Heal.
Level 18: Your responsibilities have increased with the acquisition of Dispel Magic. This spell will remove a buff from enemies and debuffs from friends. I don't recall using it extensively at the lower levels, but you never really know. Bind it to an easy to reach key (like "e"). Other highlights include the next rank of Shadow Word: Pain and Desperate Prayer (for us Dwarves and Humans anyway).
Level 20: Here we go! This is the next level where you get your second set of Priestly racials. Now everyone gets access to Fear Ward.
New Racials include:
The DPS arsenal expands yet again with the inclusion of Holy Fire. The current incarnation of the spell renders it largely useless. Once Wrath hits, however, the speed should be significantly quicker. It'll be a 1.5 second cast (I think). Priests also get their first form of crowd control in the look of Shackle Undead. This spell will become incredibly useful especially when you begin questing in the Darkshire area with the various Undead mobs running around. The next rank of Fade and another interesting spell known as Mind Soothe becomes available.
Mind Soothe effectively reduces the range at which targets will lock on to you. This becomes useful if you're trying to get somewhere fast. You can use Mind Soothe to help increase the buffer in distance that you have against mobs you're trying to sneak by. Personally, I never used it that often as I relied on my damage spells to clear the path to my destination.
That just about covers it from levels 1-20! I'll delve quickly into talents.
I wouldn't advise investing any points into Discipline just yet. You benefit more from the other trees purely from a leveling stand point. Do invest in Spirit Tap right away the moment you turn 10. It's under the Shadow tree and it'll help reduce downtime you have when facing off against mobs. The next 5 talents are entirely up to you. I preferred to remain viable for instance healing so I opted for a Holy/Disc type of build. In other words, take the first 5 points of Holy Specialization to increase your critical strike chance of your Holy spells. Note that this isn't the most optimal build to go as a Priest. If you want to stick with damage dealing and leveling as quick as possible, then pour a good chunk of your points into Shadow.
In the coming weeks ahead, I'll do my best to brief you and prepare you as much as I can.
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