Google Adsense is a program that allows you to park google ads on your website. You can make an account at adsense.google.com and read about the options and possibilities. Then you can lay out your ads and give them colors to match your website. Once done, you can copy the HTML presented into your page and presto, you have google ads on your site!

It's possible that you get google public service ads for a while until googlebot has crawled your site to determine what it's about. It reads the page the ad is on and then it places appropriate ads alongside with it.

Some bloggers have a lot of visitors and have managed to earn a few thousand dollars a month with adsense. You usually need a few thousand visitors each day to make that kind of money but that's not impossible, just write good information about something you know about, make sure other sites link to you and send you visitors, make sure people have reasons to return because you offer new information that interests them on a regular basis. Then you will see increasing results and you'll have a lot of fun doing so!