If you are a Satanist, what do you really believe?

LaVey Satanists basically believe that there aren't any gods, and yes, that means no "satan" either. As you said, they call our basic human nature "Satan", and the main reason LaVey started that was to piss off Christians. Basically, they believe in doing what you think feels good, as long as you aren't actively harming anyone else. Unless that person actually wants to be harmed. Then that's ok.