You to visit Google adsense and set up an adsense account. It is very simple. once you open your account you can then choose what kind o ad you will to appear on your forum. You will be given a code which you can add to your html coding and your ads should be live rather quickly. If you do not know much about html coding I suggest you consult your webhost, because if place the ad wrong it will distort your pages.
Or maybe you can search google for a mod to add your adverts to your pages... There always ad mods available lol