Your first step is to identify what will make your book attractive to readers. You need to think of a unique take on the subject you are writing about or it won't be worth reading. Also, developing a strong and interesting main character is key. In my opinion, a dynamic, flawed character is the most important part of a story. Start by writing about the character (this will not be part of the book) so that you know who they are before you start putting them in situations in which you don't know how they will react.