really easy, just set up an account with google adsense. create a few ads in the admin area then copy the script to the site directly or add them into widgets if youre using wordpress etc. what platform are you using for your blog?
If you don't have an adsense account just start a blog on blogger (preferably one that is related to your niche), add original content and apply through blogger. Once you're approved you can put your ads in any site that stands in the google adsense TOS
just follow what ron says. build a decent looking blog with some unique content, apply for adsense when ur blog is at least 2 weeks old, GL
Good advice. I'd also recommend that you have a lot of high quality, original content on your blog before you apply. It will give you a better chance of being approved.