Google require you to have a website or a blog before you can sign up for an Adsense account so if you do not have an account, then create a blog at www.blogger.com (free). When you created your blog, use your blog URL when you sign up for an AdSense account.
Once you get your account. You can log in to get your ad code which you put on your webpagesto show AdSense ads. There are different style of adsyou can choose from. You can even change the coloursaround.