If you don’t deactivated the function for autom. password it was given by the system and you received an eMail with that password as a welcome notification. Please take a look at your eMail account.
If you don’t find it anymore you must set a new password. Therefor you need to sign out and ask here for a new password („Forgot your password?“, link under login). Once you changed the password you could login with your eMail address and new password without being online at facebook.
