Can you tell us a bit more? How long is it supposed to be? Will you be presenting it out loud? And, can you think of any frightening experiences you've had?

One suggestion: there's no reason why it has to be terribly frightening. 

Also, does it say anywhere that it has to be a true story?

One final thought. Often, instead of trying to describe your feelings, it works better to show your feelings simply by describing your actions, what you physically did, your physical sensations, and the things that were happening around you.