Enjoy the leveling first and foremost. I ruined the game by leveling my first character entirely through the dungeon finder up until level 70. Read quest text etc, explore zones, because the first time is the most fun you'll ever have with it. So if you buy Warlords of Draenor then I'd say don't use the level 90 boost until you've leveled a character on your own.
If you're not aware the end game of WoW is comprised of 2 parts (pretty much). Raiding, where 10 to 30 people get together to essentially take on an extremely long dungeon (because of this you've got a week to get through it all) and PVP where at the higher end you and either 1, 2, or 4 people will go against another team of the same number of people, with the objective being to kill them.
I'm gonna assume that you'll put more time into Raiding simply because that's what most people do. You've got 3 roles in raiding tanks, healers, and DPS (damage per second). Tanks are your blokes with sheilds whose job it is to get stuff attacking them, and then use skills to take as little damage as possilble (for example a protection warrior will build rage to use Sheild Block or Sheild Barrier consistantly). Healers heal everyone in the raid. And DPS do the brunt of the damage so you can actually kill things.
If you stayed on a Warlock then you'd have to be a DPS, because Warlocks don't have the specializations to perform the other roles. You've got Affliction which is all about putting damage over time skills on whatever you're trying to kill and letting those tick and bring them down, because of this Affliction warlocks are best in boss encounters where you have a lot of enemies. Destruction is all about sending out spells that just crit for insane damage. And demonology (I've had to look this up since I dont know the spec that well) is apparently good at both single and multi target damage and has good burst damage for when you need it.
Despite all this it doesn't matter too much what you play while you're levelling, and it only matters when you're at the level cap if you're really trying for the most damage possible in the shortest amount of time.