Demonology is a strong choice as well; however, destruction generally pulls ahead in the early stages of leveling more so than the other two specs. The primary reason for this is that DoTS are efficient at single-target mob killing, which is what you'll find yourself doing more often than not early in your WoW career. Instead, you'll mostly be focusing on direct damage attacks. Even though destruction doesn't have the benefit of fire-based abilities yet, it does have Bane, which is a massive reduction in Shadow Bolt's cast time.
As you progress further, Backdraft is another key talent that significantly increases your killing speed at lower levels. Mobs in early WoW all have relatively low health and die in just a few direct attacks regardless of your spec, so the strength of those attacks isn't as important as the speed in which you can use them. In that respects, destruction is just miles faster.
When it comes to choosing which talents to take, it's all rather intuitive. As you go down every tree, there's a strict path of basic damage-increasing talents that don't leave much choice in where you can branch off. The only times that you can waver from the obvious is when you're faced with miscellaneous utility choices that don't impact the overall experience all that much.
The one thing to remember is that both affliction and demonology both excel at massively large pulls of mobs you can AoE down through the use of pets and spreading DoTs around. Several of their talents and skills are proc-based or focus on an area, so should you choose to go down that route, be sure to keep that in mind and capitalize on your strengths.
Choice (or lack thereof) in pets
A side part of leveling as a warlock is your choice in pets. The reality is that there isn't all that much of a choice when it comes to which pet that you choose to use. It all still boils down to the same principles that are used in raiding. Affliction and demonology both favor the Felhound for single-target DPS, while demonology defaults to the Felguard for AoE. Given that you won't always have a full round of DoTs on every single target that you come up against, defaulting to the Felguard as demonology is usually the superior choice. It offers great control abilities, and the damage is still really stellar.
Destruction still favors the Imp, and not entirely because of the talents that support it. The Glyph as well as your first foray into demonology talents when you get high enough further boosts the focus on the Imp for destruction. The one downside in all of this is that there is only of limited use for the Voidwalker. When coming across specific elite mobs that are a particularly tough to take down, it can make a suitable tank, but threat is going to be a major issue. Only resort to using the Voidwalker if you find yourself without any other option, because it's going to be a long, slow process where you'll spend a majority of your time waiting on aggro to build up.
Outfitting for success
Last is a matter of gearing. As a cloth wearer, your options are far more limited than others but also more direct. In terms of stats, intellect is the best that you can find, but secondary options are really inconsequential. Regardless of spec, haste is always the best leveling stat that you can go with. Speed is everything. Crit just doesn't hold that much value in the overall leveling process, and you won't encounter mastery until much later in the game. Spirit should be avoided at all costs, but if it has much higher intellect values and a priest isn't in the group, then feel free to take it as well.
I also want to say that you should try and be kind to the other hybrid casters at the earlier levels. Blizzard has made many great strides in low-level itemization, but certain items such as mail and leather caster pieces are still rough to come by in certain slots. Don't get too defensive should players who normally need these ask to roll against you. Be free to say no, and do tell them that they shouldn't be rolling without asking first, but keep in mind that they often struggle to find pieces just as much as you do -- that is, of course, until level 50. At that point, they should be clad in their personal armor class for the bonus.
Blood Pact is a weekly column detailing DoTs, demons and all the dastardly deeds done by warlocks. We'll coach you in the fine art of staying alive, help pick the best target for Dark Intent, and steer you through encounters such as Blackwing Descent and The Bastion of Twilight.