Writing a fictional story is good because you can make it your own. Read what your teacher gives you and continue off of that. To get a “perfect writing” I would say just make sure it makes sense to what the first paragraph talks about. Other than that, there isn’t much to prepare for since the topic is unknown. Just don’t stress too much!
In the last paragraph, you want to make sure you emphasize your point, as its the last thing the reader is going to remember from it. You may also want to include your own opinion if it is necssary.