You are starting a Warlock? Meaning... lvl 1 and up?
I'd say then that you should get to 70 and then ask about raiding...
In any case: from lvl 1 to 49 stick onto affliction
from lvl 50 to 70 full demonology and then again up to affliction. Let the felguard do the job.
Early raiding: Heavy Affliction, either up to UA or at least to SM and then some destruction (up to Ruin if you didn't get UA).
The reason for this is that bosses are considered lvl 73 so you'd need a 16% hit rating to cap... 5 points in supression give you a massive 10%.
From then on... (myself unexperineced there), there seems to be a consensus on 0/21/40, sacrificing succubus and blasting away SB. However, as a notable exception, Oakenwrath has shown also that raiding with the Felguard can be a viable option.