Write! I know, sounds obvious but if you want to be a writer, you have to develop your "writing muscle." I have written literally millions of words over the course of my career so when I turned my attention to fiction, the basic skills were there - grammar, sentence structure, story mapping and of course basic word processing.
Write what you love to read. Read as much as you can and find a supportive group of fellow authors with whom you can barter alpha reading, beta reading and tips about the business side of writing.