Ashley, your teacher wants you to write a story that YOU make up.
This site may help you.
What I do to help students write stories is give them a basic idea of where to begin.
For students who really have trouble, this has been the easiest way. Do not begin thinking about your story yet. Instead, answer these questions:
1) Who is in the story? Again, do not decide what your story will be about yet. Just find some characters. You might even pick people you know if that's easier for now.
2) Where is the story? Again, don't think about the story yet. Just pick a place that is specific. A good example would be the moon or at the Sear's Tower. A bad place would be America (too general).
3) Now that you have characters and setting, you need to identify a problem. If it's on the moon, maybe the space ship stalled of the moon is on a crash course with another planet. Try to find a bad situation for wherever you are.
Now, you can begin to write your story. Think of it in 3 steps:
1) The beginning: where you introduce the characters and the setting. Note that the story doesn't have to begin where you said your main setting will be, but it might be easier if you don't feel confident yet in your writing. Once we understand the characters, we move on to the next part.
2) The problem. How does the problem come up? How does it happen? Spend a lot of time with them in the problem.
3) The resolution. How do they solve the problem? Be creative here and don't be afraid to be unrealistic, yet logical. Have fun with the solution.
I think people get stuck trying to think of a whole story at once. If you break it down to different parts, and decide things before you start writing, it makes it a lot easier and drives the story forward.