Just go to www.commissionjunction.com
and read their description. All you do (once you're registered) is choose which companies you want to advertise for- choose what kind of ad you'd like to place on your site, copy the code and add it to your website. They have a place on your user page where you can see how many clicks you've gotten, and how much you've made.
They send you a check whenever your earnings hit $25
It's pretty low maintenance.