How many ways are there to offer advertising on a website? This could become a complicated blog post if I tried to cover my whole list of options descriptively, so I will keep the list short and simple. Just remember, these are the simplest and easiest offers you can make to an advertiser.
Google Adsense
You would create an account at Google Adsense, grab some code, place it in a widget on a sidebar. Ads would magically appear, relevant to your content, but not of your choosing. When someone clicks on an ad, Google deposits a little money in your account.
There is a way to use Google to place ads from specific clients onto your website, but they have to have set up a Google Adwords account and created an ad in that account. When someone clicks, the click is recorded on Google when the client pays you and Google a fee. No click, no fee.
Private Commissions
You could verbally negotiate with a vendor. For a fee, you would place an ad somewhere on your website for a fixed period of time. There would be no guaranty to the client. He might get nothing from the ad. With Adsense, no one pays unless someone clicks. These are hard deals to negotiate and require substantial traffic to your website to make attractive to any client.
Crosslinks
Ultimately, an ad on a website is just a crosslink. The cheapest and easiest way for everyone to advertise on other people’s websites is to exchange links. You put a link on your website if the other guy will put a link to yours on his website. No money exchanges hands, but the opportunity for both of you to increase your traffic is traded. If you think that your website link is worth much more than his, then you may want to negotiate a click through rate, and setup the tools for tracking that and getting paid.
Conclusion
Adsense ads are easy to install and maintain. Crosslink exchanges are easy to employ and maintain. Selling ad space and managing click through traffic for individual clients is a larger project to setup and maintain. More on that some other time. I wanted to keep this answer simple for now.