Anyone can write.
As a teacher, I believe this. But the truth is, while anybody can write, not everyone can do it well. Many novels shouldn't have been published. Where's the quality control these days? Demand better, folks.
Authors write what they know.
If everything I wrote was based on real-life experiences, I would be precluded from dabbling in speculative fiction genres. I write what I've imagined and fully conceptualized. I create characters and drop them into worlds I've built and explored. I know my characters inside-out: their joys and sorrows, loves and losses, victories and defeats. So I write what I know.
The first novel you write will not be your first novel to sell.
I'm still querying my first novel (after multiple edits and revisions). I'm also querying my second, third, fourth, and fifth novels. Novel #6 is the first to greet the masses this year: Captain Bartholomew Quasar and the Space-Time Displacement Conundrum. Not because it's necessarily better than novels 1-5, but because I've had much better luck with small presses than agents.
You will never sell everything you've written.
I don't submit work unless it's fit to print. So far, that's been 122 short stories, novelettes, and novellas (108 accepted for publication) and 8 novels (1 accepted for publication). With enough patience and polish, I'll eventually sell everything I've written to a publisher. My only self-publishing is re-publishing when the rights revert to me. Why get paid once when you can twice?