Your horror story must have a setting, characters, and a descriptive plot, just like a full-length story, so do some brainstorming:
WHO - Who are the main characters? How are they described in the story?
WHAT - What is the scary story about? What is the problem?
WHEN - When does this story take place? What is the time of day, time of year or season, and is it set in the past present or future?
WHERE - Where does the story take place? How is the setting described in the story?
WHY - Why is the main character in danger? Is it resolved?