Hi. Writing a story is very personal and you should use your imagination and your ability to develop a text. For that reason, I can't write a story for you, but I can help you by showing you how to write a story and that's what I'm going to do below:
First, you need to think about the plot of your story and you can do this by deciding what you want to write about. Do you want to write about a new kid who has arrived at school? About a girl who falls in love with her best friend? How about a group of friends who live an adventure in an unknown land? Any story that your imagination wants to develop will be welcome.
Next you should think about the characters. As your question asks for a story of 300 to 500 words, it would be important that you think of a few characters, as this story is small.
You can even make a character the narrator of the story and everything is told from his point of view and with his perceptions.
Here is an example plot that you can use as a model to write your story: You can write about a boy who is studying to be a great illusionist. He wants to do magic tricks that surprise his family members at the next family dinner. He spent several weeks practicing magic tricks. Will he be able to convince the family that he can be a good illusionist?
You can find more information on writing a story in the following related question:
