What one bit of advice...?
There is only one good reason for writing fiction, and that is for the love of telling the story. Getting published and getting paid for your work is enormously validating, of course, but if you don't love telling stories, there are plenty of other ways for someone with good communication skills to make a living.
If you ever find yourself in a position where you are writing a piece of fiction solely for the money: stop right there. Don't do it. There's already an adequate supply of indifferent hackwork in the world. Don't add your contribution to that vast, fetid heap.
What's your one piece of advice?