And you'll want to check out their Program Policies, too, to make sure your site is suitable before you apply.
One very important guideline is that you're not allowed to encourage your website visitors to click on ads; Google is clever enough to detect suspicious click activity on ads on your site, and you can be banned from the Adsense program if you click on your own ads or ask others to do so.
Your site needs to be of good quality - Google stresses the importance of having your own, original content on your site. You can't just copy chunks of stuff from other sites, stick it on your website and pop some Adsense on the page!
It's not Get Rich Quick
Turning your website into a million dollar a day generator that runs on autopilot... I wish! All the webmasters I know who are earning good money with Adsense do so because of one "Adsense secret" that really isn't so much of a secret after all - hard work!