For enrolling to the Google AdSense program, go to https://www.google.com/AdSense and click on the Click here to Apply button. You will be presented with an enrolment form where you need to fill-in some details like your website information (URL and website language), the Google AdSense products that you wish to use (i.e. AdSense for Content or AdSense for Search), your contact information, your email address and your preferences. You will also need to select an account type (business/individual) for your AdSense Account and agree to the various policies, terms and conditions of Google AdSense program. The account type will not affect your revenues in any way; it is there just to determine if the payment is to be made in the name of a company or an individual. The email address and the password that you provide during the enrolment process will be required by you for logging into your account later on. Note that you will be referred to as publisher by the Google AdSense program; so when you return back to the Google AdSense website, you will be asked to enter your login details as an Existing Publisher.
Thats it. You can now submit your application form for Google to evaluate and get back to you.