Write what you know. Pretty cliché advice, but you’re going to have an easier time making characters cobbled together from people in your own life. Or situations you’ve experienced. Whatever you do, don’t try and be an expert in an industry you haven’t worked in or in a subject you haven’t studied. Nothing worse than feeeeeemale scientists in stories who boobily bounce their boobs to the lab where they make their dead dad proud his daughter became a physicist just like him.

Why now, why these characters? The fundamental part of storytelling is answering why the audience should care about this story you wrote. For example, we care about Luke Skywalker because he’s going to be at the forefront of a successful rebellion. Nobody wants to read about Luke hanging out with his friends picking up power converters while the Empire continues to carry on without a rebellion. Why is your story taking place when it does? Why are these characters important to this situation?

Conflict is inherent to good stories. What do characters want, materially or socially, that puts them at odds to the wants of other characters? Maybe it’s the same romantic partner. Or the same job. Perhaps they’re on opposite sides of a war. You can have a story with no villains and remain a good story so long as there’s conflict between the protagonists.