(skip some example paragraphs)
Your theme, the abstract summation of your work should be objectively valid, but otherwise the choice of themes is unlimited. You may write about deep-sea diving or anything you wish, provided you can show in the work why there is objective reason to be interested in it.
The most important element of a novel is plot. A plot is a purposeful progression of events. Such events must be logically connected, each being the outgrowth of the preceding and all leading up to a final climax.
I stress the word events because you can have a purposeful progression of ideas, or of conversations, without action. But a novel is a story about human beings in action. If you do not present your subject matter in terms of physical action, what you are writing is not a novel.
Let me give you some examples of the difference between theme and plot, starting with my own works.
The theme of We The Living is: the individual against the state, and more specifically the evil of statism. I present the theme by showing that the totalitarian state destroys the best people: in this case, a girl and the two men who love her. When I say that the story concerns a girl under a dictatorship and the men who love her, I am already talking about the plot.
Incidentally, if one names only the most general meaning of We The Living -- the individual against the state -- one does not indicate on whose side the author is. It could be a communist story showing the evil of the individual, but then the plot would be different. Or it could be a Naturalistic novel, a presentation of life under a dictatorship with no moral sides taken, The theme, however would still be: the individual against the state. So when you work on a story of your own, make sure you define your theme clearly. That will help you judge what to include.
From The Art of Fiction by Ayn Rand