Before I write a story, it rains in my brain. In other words, I brainstorm the story's details. The details fall like raindrops, eventually puddling into the parts that every story needs--the beginning, middle, and end.
To brainstorm, I'll ask myself questions like...
"What is the main character's problem?"
"How does the problem affect the main character/the main character's life?"
"How does the main character respond/react to the problem? What actions does he/she take?"
And, after answering lots of little questions like "Does the story take place over a day? A week?" or "Are there other characters who help the main character?" I'll finally ask...
"How does the main character solve the problem?"
Like rain feeds the earth, the answers to these questions feed my story. They are details I use to grow a plot and characters, and I let them fall until I know my story's beginning, middle, and end. Then it's time to dry off and start writing.