You simply cannot monetize your forum if it is completely new and have empty categories. When you have decent amount of traffic along with thousands of active threads than it will be a good idea to monetize it. You can use any ad networks like shareasale, cj, clickbank etc to monetize your forum. Some PPC networks like Media.net, Infolinks also works great.
Forums need vast amounts of sustained traffic, It would be very expensive to launch through paid ads.