Ask your instructor for the url of your WeBWorK course. Usually this is a path to a server on your campus. Additional documentation for students can be reached via the "Students" link in the column to the left.
Complete and submit the following form to request a user account. Make sure that you first read the Terms of Service before requesting an account.
Once the account is approved, you will be e-mailed a notification message and the account will be usable at login.