hi i made a belf warlock and im wondering what type he should be because im going to be doing lots of dungeons and i herd destruction warlocks run out of mana easy and quickly but have powerful spells so idk, whats ur opinion

Warlocks don't use Spirit... AT ALL. Disregard the other guy...

Mana is never a real issue as long as you stack Intellect and spam Life Tap (Converts HP to Mana).

Destructions offers heavy hitting spells which is decent for most encounters, Demonology offers you one of the best AoE style fighting via Felguard and Hellfire spam... Then as for "Affliciton" it's not really taken too seriously at high levels as it requires a lot of upkeeping the dots and takes time to dish out any decent damage.

(So imo just go Destruction.)