It’s quite simple really… you want to level and level as fast as possible. First, go get QuestHelper and follow it. When you get to the “Kill X of these mobs”, this is what you do:

Talents

You need to spec Affliction. To make this quick, the talents you need are quite obvious but just-in-case, here’s what you must pickup as you level – Imp. Corruption, Imp. Life Tap, Soul Siphon, Fel Concentration, Grim Reach, Siphon Life, and Dark Pact.

Note: Don’t use immolate! You’re specced Affliction and none of your talents help with it. Besides, it’s just a mana waste – at this point you’re already over killing the mobs. More...

1. Warlocks summon demons!
   The first in the list should be the coolest reason. How cool is it to be able to summon demons to do your bidding!? Especially demons that are actually useful while playing. The felguard talent pet is a beast, while the felhunter pet is great for PvP against casters.
   
2. Curses, corruption, and fear
   Even though summoning demons is very cool, warlocks can throw curses and cause fear in the enemy. The curses aren’t wimpy either, they’re devastating! Through the use of curses and corruption, warlocks cause tons of damage to their opponents; causing them to die a slow, gruesome death. Curse of tongues makes baby paladins weep. More...