Register on Google Adsense site, then post their javascript on a widget on your blog. It's easy as that, start earning.
People read left to right, so post them on the left, middle, and top section. Make sure you blend in the ads really well so they look part of your site. But make sure you have a good link color, so if you have a regular link color is blue on your blog. Make sure the links on your adsense unit are blue.
Make sure you follow all the adsense rules or you will be banned. If you're going to have a title over it, make sure it's only "Sponsored Links" or "Advertisement." Anything else will get you banned. And don't align images next to it.
Best add units are 160x600 and the Large Rectangles (336 x 280) or (300 x 250)
