First, look to the upper-left of the screen and find the link that shows as "Login" and click that.
Don't choose "Register" if you know you've already made an account, even it's been a long time since you've used the account last.
Second, we have a link below the fields for logging in normally that says "Forgot your password?", click on that.
Now, you'll be taken to a dialog that'll ask for your username or your email address. Either one will work.
And lastly, you'll be given the option of having a new password emailed to you or using your Secret Question to access your account. Choose the one you feel best with or have access to.
If emailed, your username and instructions on resetting your password should be sent to the email address you used to register to the forums.
If you no longer have access to that email address, use the Secret Question to gain access and contact a staff-member pertaining to the situation. The Secret Question Password Reset should look something like this.