Hi,
Well, do you have a website that gets lots of good taffic? Is this what you are looking for, to sell advertising on your website? If so, you need to first of all make a list of the kinds of advertising you want to sell. Do you have subscribers to your website newsletter or anythign like that? Do you have a top banner adn a bottom banner on your site? One way is to add a banner at the top, and underneath it in small print write "Put your banner here". Better yet, create a link to an advertising page, and put together a page that shows what you have to offer.
Have a look at various forum sites and see how they do it. Tht will give you lots of ideas. Of course, you will only make money if you have good traffic, and if you have that then that very same traffic will likely buy your advertising. Otherwise you will need to promote your advertising opportunities, which you can do also. Good luck!
Paul-Marc