There's a few tips which I think would be really useful.
1) Before jumping into ideas, have an idea for a main character and try to develop it (your character doesn't have to be some 'immortal-like' person who saves the world, or some exaggerated idea like that; often, the simple characters with an insightful meaning will be the most effective
2) Have a setting for your character; what time period, where will the story take place, what people are around them
3) Experiment with different perspectives; first, second and third and settle with the one you are most comfortable with (personally, I liked to use third person)
4) Have a structure to your story; you don't need to get through a whole lot in your piece, as you simply won't have the word limit nor the time to do so; even a simple structure like below can be really effective
- Inciting Incident
- Rising Action
- The Conflict
- The Climax
- The Resolution