which subject you have to choose? I guess, something which you're good in? You don't want to be an owner of a linux forum when you don't even know what it is.
SEO and Programming can come under the webmaster forum category. General Topics can come under a general chat category. Taxation could be part of the general chat forum. Like AMiRU said, it's pointless creating a forum if you have no interest in that subject. Choose something you're good at and passionate in - Then away you go.