MyBB Community Forums

paid for them if someone visits them?

Like Google Adsense maybe?

:o I never knew google did ads.
does it tell you how to set it up on mybb?

You can put ads in Mybb forum. They already have a Mod for this:

http://mods.mybb.com/view/ads-after-first-post-2.0

This will help to display ads after post.

You can also add ads via editing the templates.