From the Blogger admin page, you'll find a tab on the left called "Earnings." This lets you link your blog to your Google AdSense account. AdSense automatically detects the subject matter of your blog posts and displays relevant ads to the reader. You then can earn money (usually a few cents) per click or ad impression.
You can customize where AdSense ads appear on your blog, change the font and colors of text ads, and restrict some ad categories from appearing on your blog. Once you're signed up and your ads have been active for a few days, you'll see statistics -- including how much you've earned -- in the "Earnings" tab.