The problem with warlock in my opinion is that you need at least 20% haste (I cannot vouch for affliction), or you will have an awkward time. Demonology especially needs haste, because you have to hard cast virtually every spell, and more haste equals more imps, which means better Thakiel Consumption hits (steals health from each active pet for a nuke, more pets, more damage). Haste smooths everything over.
Demonology needs the more haste you can stack , and its hard to get it, get the items with the haste pray you get titanforged, that means you almost never get upgrades.Gameplay is very boring,you are a stationary turret with no defenses, its ok on single target raids , mediocre in dungeons, absolutely awful in pvp .
Aflliction need even better gear to start to shine , 30% haste is required and as much mastery you can.Great to do aoe in mythic dungeon, good in pvp, awful in raids.
Destro 30% haste and as much crit you can stack.Good in pvp, mediocre in dungeons, mediocre in raid except when you can cleave.
Demo is good at single target, affli at aoe , detro at cleave, that means a considerable amount of ap to farm to get viable in all situations, no class has to do this , even when you max it, it isnt specially good.
Theres a lot lot better to pick,and more fun, i'd suggest a mage a shaman or a druid instead.
I highly suggest new players to pick another class, you've been warned , i wasnt.