If you have a gmail email account, then you're already set! Just log in, and click "Documents" in the top left navigation bar to access Google Docs.
If you don't have a gmail account, you can create a New Google Account by going to google.com and clicking "Sign In" in the upper right corner.
Under the sign-in box, there is an option to create a new account.
Choose a username (can use a non-gmail email address) and password, and some basic information. Read the terms, and click "I accept. Create my account"