Satanist is a broad and varied category: some are theistic and believe in the existence of a literal Satan, while others use Lucifer as the original symbol of resistance and bringer of enlightenment to humankind. Much like any religion or worldview, there’s a huge variety in what Satanists believe (and how scary they may be). A good place to start would be learning about mainstream American satanism as practiced by LaVey’s Church of Satan or The Satanic Temple. Or enjoy the OG Satan that inspired the religious movements by reading Milton’s Paradise Lost!