If you have forgoten your username, you can retrieve it by clicking on the log-in button and choosing Forgot your UserName? from the links at the bottom of the login page
As long as you can remember - and still have access to - the email address you used to create the account, your username will be sent there.
If you have forgotteen your password, it's a similar routine. Click on log in and choose 'Forgot your Password?'
Again, so long as you have the original email address, a password reset link will be sent to that address so you can reset the pasword. You can reset it to the same as before, up to 5 times, then you must choose a new one.
As you will probably guess, if you forget both username and password, you can still fix it - you just need to get your username, then use that to reset your password.