Your username is the email address for which the account was created, e.g. firstname.lastname@example.org.
If you forget your password, you can request a link that leads you to set a new one.
1. Press the link "Forgot your password?"
2. Press "Send instructions"
You will get an email in your registration address with a link you can use to set a new password.
3. Type in a new password and press "Change password"
You are redirected to the login page and you can log in normally using the new password.