Hi,
I personally always played Affliction during leveling. Starting a 30/0/21 build. I think that's the most reliable spec in term of survival, dps and fun.
Some people like to play in Destruction build, but I don't recommand it in low level, this is an awful mana eater.
And about Demonologic spec, I really never played it (except in Soul Link build when 60 in PvP) so I couldn't tell you about it.
What I'm doing now is to get talents until Siphon of Life, then putting points in destruction tree to reach Ruin, and then finish the Affliction tree until Shadow Mastery (get 10% shadow damage before 55-60 isn't really worth it in my opinion).
I'm sure other warlocks will be posting more advice and other playstyle, and you can always browsing all the thread about Warlock specs
have fun !