If you already have an account- simply log in with your email and the password you created then click on the tab: enroll in class. Add the new ID and password.
If you do not have an account- Go to turnitin.com :) Click on sign-up, then student under sign-up again. Fill in all the blanks including the class ID, password, email address (make it a real one you use- I sometimes send reminders), make up a password etc. Once you are enrolled you just sign in with your email and the password you made up. Submit papers by clicking on a posted assignment and either attaching or copying and pasting your document. :) So when your printer doesn't work-- I still have your work! :) Many colleges use this too. So it is good practice.