My biggest advice is to have low expectations, but I’ll save discussing that for another time.
My next biggest is to start small. I see so many first time authors start with a multi-chapter story. And their first chapter sucks. Then they are committed to a major project that’s going to have a small and shrinking audience. I was going to do that and fortunately decided to start small before publishing the first chapter of my magnus opus (turns out the first chapter of my magnus opus sucked). Your first story is not going to be very good. Write something small (2-4 pages) and get your feet wet.