To start with, I would like to let you know that you'll probably get a more qualified response at PromotionTalk.com forum, but just to give you an introduction, almost all companies get paid by advertisers aor advertising agencies they deal with, who pay them commission.

They can either get paid by the number impressions, clicks, sales, etc. There are plenty of advertising networks available for publishers, but to get qualified in the best, they'll most likely look at web site's popularity, content and other features.

Then they provide you with a code you should place on your site and you're done - you'll be able to monitor your revenue results in real time through the publisher's control panel.

You can also go alone and find advertisers on your own by dealing with them directly. In this case you need your banner management system set up, merchant system to accept payments, as well as improve and keep up the quality flow of content. The return you get is bigger though, but the headache is bigger as well.

Hope this helps.