@Imagine_Dragons4Life If you really want to write a book, I suggest coming up with the genre you want to write first.
Then, start thinking about more specific details. Who are your main characters? What’s the plot? If you can’t come up with anything, you could always go on Pinterest and use a dice roll suggestor to come up with ideas! After you’ve come up with a basic plot, write a simple outline. What’s the introduction, resolution, climax? It seems annoying to write but it will ensure you don’t have to stop midway. And finaaaaally, you can start writing!
Hope this helps <3