You can integrated your site with some monetize program, such as adsense, link exchange, put banner, and other method.
If you don't like adsense, I recoomend ReachJunction.
For new sites, I have found zenoviaexchange.com and madadsmedia.com both accept just about anyone. I use them and I have found their ads to be clean and they have paid.
Yahoo! Advertising Network
Microsoft Advertising adCenter
Microsoft Ad Network