you don't - but i've seen your other posts and i think i know what you're asking.
Forums are either the whole messageboard (as in "Aquaria Central is a forum for people to ask questions about their fish") or the specific categories - Freshwater Newbie Forum, Catfish Forum, Cichlid Forum, etc. Threads are individual questions/lines within a forum, and posts are the individual responses.
I think you want to know how to delete a THREAD, and so in that case you would contact a moderator (their names are in red), ask them to delete it, and explain why. If you want to delete a post that you have made, either do it within the time frame allowed (there is an "edit/delete post" button) or again, ask a mod to remove it for you. They have the power.