Read. I taught myself to read when I was three, and I've been reading obsessively ever since. M.R. James, Shirley Jackson, Dan Simmons, Donna Tartt and Stephen King are high up in my pantheon of gods. Then do your research. Find an idea that speaks to you, there's sit down and write. It doesn't have to be perfect; you can always go back and revise. Keep at it; writing is like any other activity: the more you keep at it, the better you become.