Try reading other short stories. You can find these on the internet, or in books. Even just reading novels is helpful. Most of the time, markers are looking for how you shape meaning - ie, how you use language forms and features (metaphors, similes, structure etc) to make a point.
Try writing as often as possible. Dont worry if its not great at first - youre the only one who will see it. After a while, you will see your writing improve.
If you still need more help, see your teacher. Dont be embarressed to ask - its their job to help you. If you hand practise stories/essays to them, they can advise you one how to improve your writing.
The first and largest hurdle is confidence. Motivate yourself, and dont ever think that you cant do it because your "not smart enough" or something.