Here is a small introduction provided by Google into what its AdSense program really is:

What's AdSense?

The flexible, hassle-free way to earn revenue online:
  • Get paid for displaying targeted Google ads on your site.
  • Customize ads to match your site's look and feel.
  • Track your success with online reports.
  • It's free! With AdSense, you'll pay nothing, spend little time on set-up, and have no maintenance worries.

Keep the following main rules in mind.

  1. You must have sufficient original content when you apply, don't apply before you've built a large portion of your website.
  2. Do not click on your own ads.
  3. You can place a maximum of three ad units, three link unit and two search boxes on a page. Since these numbers may change read the most up to date numbers here.  
  4. Do not place adult content on your site.

So when you're ready, apply online with Google's registration form (here)!
For help on how to place AdSense ads in your website, click here (this is a link to another page on our website).