Plan it all out first. Draw mind maps - start with who is the story for (age/sex). Then start on a plan for theme, main characters, when it is set, etc. Then write a short synopsis (imagine what would be on the back cover). And build all the ideas into your story. A story should always have a clear beginning (introducing your characters and the feeling of the story), a middle (where you build it up), and the end (pull it together morals of the tale, what was everyone's fate, good or bad). Give it a go, and when you feel ready, share it with a trusted friend who can give any constructive critisism.