To brainstorm a short story, you could try freewriting. Just jot down whatever comes to mind related to the idea you have. Also, look at other stories for inspiration, but make sure to put your own twist on it. Another thing is to talk to people and get their perspectives or experiences that might fit into your story.
The key to brainstorming a short story is to be open-minded. Think of unusual combinations or unexpected plot turns. Maybe start with a random object or event and build a story around it. You can also create a character first and imagine what kind of adventures they might have.
Well, start by thinking about a theme or topic that interests you. Maybe it's a place you've visited, a person you know, or a problem you've faced. Then, let your imagination run wild and think of different scenarios and characters related to that theme.