Building a community is difficult - you need stickyness and patience. If your content-based website is regularly updated with new, fresh and unique content, then adding an (initially) small forum for discussion *can* give good results over time. Lots of time and effort are needed if you plan on doing this on your own though. See the forum community building [webmasterworld.com] section for more indepth discussion of this.
As for AdSense for forums, short answer: it isn't ideally suited. This thread [webmasterworld.com] should sort you out though.