To retrieve a forgotten login use the forgotten login link posted above.
Once you know the login, then you need access to the registered email.
If you know the registered email and still have access to it, it is simple to reset the password through the other link posted above.
If you don't know the registered email or don't have access to it, you would need to do a 'recovery' on your account.
A recovery is submitting proof that you created the account. Jagex need this proof before they will make a change as important as changing the registered email for a player who doesn't have access to the current email.