Your username is usually your email address.
If you joined online then your username should be the email address you entered when joining. You will have received a confirmation email regarding this on the day you joined.
If you joined via your school then you should have been allocated a username which will resemble an email address e.g. user1@myschool. Your department will have received a letter outlining your school's usernames when you joined.
Any problems please contact support@teachit.co.uk or call 01225 788850 during office hours.