So You Want to Write a Book?

Most of the posts on this blog are geared towards writers who have been writing for a long time. But as I was drafting another post, it got me thinking back to when I was writing my very first fantasy short story--a short story I then decided actually needed to be a book. At the time, I had never written a book before and I had absolutely no idea where to start.
What were the rules? Was I supposed to write a certain way? Make characters do certain things? Follow a specific plot? Heck, I h...