You'll be prompted to enter a password of a certain length and complexity based on the help desk's password security requirements. Once you create a password, you can log in and start using the help desk.
You can always change your password later by logging in to the help desk and updating your user profile (see Changing your password).
If you forget your password, just click the Login link (located above the menu bar) and then select the Help! I don't know what to enter here! link. You'll be prompted to enter your email address (it has to be same address you used to sign up) and you'll receive an email that contains a link to create a new password.