The first rule, is, of course, the most important, and it is truly, I think, the defining line between a good a bad novel. (Grammar aside of course!)
Rule 1: Write what you love!
The reason I say this is the most important rule in writing is because, who wants to write a story about something that does not interest them? Write what you like to read. I, for example, love history and historic fiction, and Scotland, so I write a lot of Scottish historic fiction. If you enjoy reading fantasies or mysteries, write those! If you happen to enjoy two different genres, like, romance and historical, then write a historical romance! Or historical mystery, or a mystery fantasy or whatever! Just write what you love, that way, you will be sure to put your whole heart and soul into your books. The choice of what to write, is the one thing that separates creative writing from the assignments you have to do in school and stuff like that. (Now, I for one have never hated writing essays, and later, I will most likely post something about writing creative essays such as personal essays which are most of the time just as fun as novel writing!) When you write, do not fret over what other people might like to read, worry about what YOU would want to read. Too many times, I have seen hundreds of the same kind of books in stores because one authors thinks an idea is enjoyable to write and everyone ends up liking their books; but then other authors see how well the first person's books went and they decide to write something identical to them! I am completely against fad writing. If one person's books are good, then fine! But that doesn't mean that the next fifty just like it will be better! I'm not saying it's bad if the fad at the time is what you like to write, I'm just saying not to write something to feel obligated to just to get people to read it! Remember, this is not high school! No matter what people say, just keep writing!