If you're trying to login to the forum but have forgetten or lost your password, you can retrieve it with the lost password tool by following the Forgot your password? link from the login screen.
You can enter either your username or your email address so long as you check the appropriate option, then click the Send button to send yourself an email containing a link to reset your password. It's also possible to avoid the email altogether if you've set yourself a secret question and answer in your profile, but please beware of questions that anyone could answer or guess (like 'what is the first color of the rainbow?', which would be a very bad choice)!