Just want to point out it doesnt matter much how good your class is in raids. All dps classes (not hybrd) are taken, they all have uses. You will be competing with the players of your own class in the raid tho. So like warlocks do less damage than mages early on. But warlocks wont be judged against mages, but against eachother.
I'd play the dps class you find looks fun/style/whatever strikes your fancy. As for ease of leveling, hunter/lock easiest cuz of pet, mage pretty easy, rogue a bit longer.
Last thing, dont be afriad to ask questions/tips from experienced players. Ask about macros/etc.