Ad based apps

If you're not the patient type, stop reading. I have almost no knowledge of what I'm talking about so I'm asking extremely basic questions here.

I want to put an app in the app store. It's not the kind of app people would want to pay for, but they deifinitely would get it if it were free. I want to do an ad based version. How much do apps get per download using ads? How do you go about getting ads on your app?