For a F2P warlock, in my opinion, I would spec affliction. Stack as much intel as you can get, level enchanting to enchant your gear, leatherworking for armor kits, tailoring and enchanting (disenchant what you make in tailoring for mats, and make a TON of wizard oil and store it) and finally, herbing and alchemy. Due to changes in profession perks, gathering profs no longer give any benefit. Alchemy will let you use potions.
Now here is why I say affliction. F2P will literally get face rolled into the dirt vs a paid account level 29. So you want to dot and run. I'm not commenting on how good a player you are, just the sheer fact of stats a paid account can stack is astronomical vs a F2P.
Hope this helps.