Just start writing. The first 20 drafts may be horrible, but keep writing and learning. Don't give up. Join a writer's group and listen to the advice of others. Learn to "show, not tell" by describing what is happening in your book. Study books you like until you understand why you like them. Do you care about the characters? Enjoy the dialogue? Love the story? Keep writing, sharing, editing, and improving. And, again, don't give up!