By far, the #1 registration/login problem is a bad email address. When you register, double and triple check your email address. I see at least 2 out of 10 registrations that "bounce back" because the person mistyped their email address. Since they mistyped it, they never receive a confirmation email, they cannot activate their account, and I have no way of contacting them because they provided the wrong email address.
If you have received your confirmation email and activated your account, but you have forgotten your password, there is a link in the login form named "Lost Password?" Click that link and the system will guide you through resetting your password.