The first thing would be to familiarise yourself with the big players in the forum space; bbPress (from Automattic, makers of WordPress), vBulletin, Invision Power Board, etc.
Decide which one of those you are most comfortable managing... then look into the multiple forum add-ons / extensions for all of them.
One point; how were you planning on financing the forum provisioning business model ? Through advertising, or a freemium model where site owners of a "free forum" eventually go for a premium paid model? Or both?
Rob