just go to adsense site and sign up for an account. once you are approved, login to your adsense account and setup the code.
read this official blog of google adsense. http://adsense.blogspot.com/?utm_source=aso&utm_campaign=ww-en_US-et-asfe&utm_medium=link
It's simple question. Just going to http://www.google.com/adsense and register, waiting for approved. Getting code and insert into your website. Drive traffic to your website and just waiting for money. Right ?
Anhbloginic already answer your question. However, suggest go through adsense TOS before apply adsense