If you signed up with an email when creating your account, your username is your email. You can learn how to add an email to your account by clicking here.
If you don't have an email, your username is your first and last name followed by your school code. For example, if you are Harry Potter and go to school at Hogwarts, your username is harry_potter_hogwarts.
If you don't know your school code, please ask your teacher, or email firstname.lastname@example.org.