Adsense is probably the best program in the world. Part of famous Google's search engine. With Google Adsense any owner of site, forum or blog can earn some money.
Google Adsense works in PPC system. This means they pay per click, whenever a visitor on of your site, blog or form, click on the advertising of them.
Through Adsense earnings of a blogger or owner of the site, forum, will be proportional to the number of hits on his website, and quality of them.
But Adsense does not allow their ads to be displayed together with other advertising from other companies.
Another important issue would be that Adsense does not accept any site, type: pornographic, defamatory, etc.
Choose a site template that looks a bit like adsense.
Put ads in the most visible places, where the visitor first looks.
Remember that some topics will make more profit than others.
And most importantly: do not click on your own ads.