The answer is relatively complicated. It depends on the number of visitors you have to your site, any product sales you might have, along with any e-mail newsletters you might have.
Most people depend on banners to start their revenue source.
It is relatively simple and easy. Yet, the clickthroughs to banners has dropped significantly in recent years as banners have proliferated on more and more sites. Banner networks include Flycast and Burst. Another method of generating banner impression revenue is to sign-up with a company that will pay you on a per action basis. That is, for every lead or click generated.