Start with noting down what genre you want to write, what path you would like to go in, what’s the goal ( if any), what category: teen fiction, non fiction, supernatural, etc. would you like to be in. Hopefully by noting down or drafting, brainstorming, what you want to write about and how you want it to flow, then maybe start planning it, characters, descriptions, how you want the ending of the book to be, make an outline, drafting it until you find what fits for you. I hope this helps! Good luck!
xoxo PuresuasionSoul