To get started with AdSense, follow the steps below:
You should have a blog/website to start earning with adsense. If you don't, then you can go to http://www.blogger.com/ for your free blog or http://pages.google.com/ for you free website.
Go to google.com/adsense, then click the Sign Up Now and fill up your details.
After submit your application, you need to verify your email address by click the verification link sent by google. Your application status will be update in several days, and you can start to earn with Adsense once approved.