The first step would be to determine an appropriate prequel or sequel to write about. A prequel is a work of fiction that takes place before the original piece of fiction; the idea here is that after the author published the original story, he or she decided to tell what happens before the original story. A sequel occurs when an author decides to follow up their well-received story with another story, one that usually features the same characters and setting. Here are some well-known suggestions:
The Star Wars Trilogy: Lucas made episodes IV, V, and VI before he made episodes I, II, and III. In this case, the prequel episodes explain some much needed information: for example, how Luke's father became the dreaded Darth Vader.
The Hobbit, Lord of the Rings: Tolkien wrote the Hobbit first and then followed the storyline up with the much more epic Lord of the Rings.
Since you did not provide any specifics on the purpose of the essay aside from the topic, I suggest that you first begin building background on the prequel and sequel idea. Perhaps in your essay, you could compare how the prequel or sequel is similar or different to the original.