before u write a story think of the characters..
think on how u can begin it...n how its ending should be...
think of the plots...
n if u r asking this question for ur xam, thn hav tym take a decision that u would cmplete thestory within half an hour..becoz tym is limited...
improve ur speed..
Introduce your main characters 
also at xams don't forget to give it a topic...becoz it carries 1 mark..
Interesting verbs are seldom very interesting
Look for ideas in real life, such as funny quotes, interesting people, or occurrences at school.
A plot is based on cause and effect.
Plan your scenes
The story must have a beginning, middle and an end.
>Theme - A theme is something important the story tries to tell us
Story Structure - Decide about writing the story either in “first person” or in “third person.” Third-person pronouns are “he,” “she,” and “it”—so writing in third person means telling a story as if it’s all about other people. The first-person pronoun is “I”—so writing in first person means telling a story as if it happened to you.
Characters - Before you start writing, know your characters well.
Setting - Set your story in a place and time that will be interesting or familiar.
The strongest children’s stories have well-developed themes, engaging plots, suitable structure, memorable characters, well-chosen settings, and attractive style. 

hope it can help u :)
hope it will help u:)
if i helped please click the thnks button:)
i more tip - GIVE A CATCHY TITLE..
Eye catching Titile..introduce the main character / s in a interesting way rather than just telling the name..use a really nice language that will make the reader imagine the story..let the story be expressive which will make the reader understand the feelings..use is best to end the story with a suspense as it makes the reader remember the story even after he reads it.Hope it helps and pls mark as the best answer. :-)