I made use of the free level 80 a while ago, and I picked a lock back then. Thing is, I never played the class before and all those spells are a drastic change coming from a rogue. (A very minimal rotation imo). It's because of this I was wondering if there is one specific spec I should chose. I've also noticed warlocks never use pets in PvE raids/dungeons. Why is this?
I would recommend going destro at the start while getting the hang of the basic lock experience.
The reason you don't see many locks with pets nowadays is because of the lvl 75 talent grimoire of sacrifice, which kills your demon and buffs you. It is not a mandatory talent but I feel many pick it so they dont have to manage a pet, plus it is sort of oldskool feel to it.
Go Destro. Especially if you have little experience with playing a ranged class. After that I'd say Affli is easier than Demo but both are two of the hardest specs in the games to master. Destro is probably one of the easiest ranged specs out there right now.
The reason why Destro/Affli locks don't use pets is because Grimoire of Sacrifice provides such a high damage boost to single target spells while the pets do nothing to their energy regen (embers/soul shards). Pets for Demo gives demonic fury and is the main reason why they use it.