If you have not registered on this forum, you won't be able to log in because you don't have an account. Joining BMW RA does not automatically give you an account here; you have to register separately.
If you have gone through the registration process you will have picked out a username and password. If you don't remember doing so, you haven't registered. You will get an email in reply to your registration attempt; SAVE THAT and do what it says. If you furnished a fake email address, well, you're not going to be able to do that, right? It has to be a real email address.
If you registered, confirmed it, did everything you were supposed to, have your username and password, and you STILL can't get in, let us know. This basically never happens.
If you are having a problem, it is almost certain that you either never registered (this is actually very common) or that you have not followed through with the confirmation.