I tried explaining this to Steph a while ago. Basically, create a blog about your favorite subject, (Pokemon?), learn SEO and PPC advertising, keep working the site, (join forums about your favorite subject, make friends, visit their site, talk about the subject, include links to your site in your signy), keep learning, keep building, keep working your blog, until you get lots of people visiting, and the ads that appear will become related to your keywords.
First step: Learn how. The info is free online, so go for it.