I think it is a common mistake of students to take on subjects that are too big. Unless you are specifically required to write about Homer's Iliad as a whole, you ought to choose a single character or incident and focus on it. That is what many great writers have done. For example, Shakespeare wrote an entire play about Troilus and Cressida. Achilles is a very interesting character. If I remember correctly, Homer introduces the epic poem by asking the muse to help him write about the wrath of Achilles--that is the Greek hero's reaction to the death of his friend Patrolclus and the hands of Hector. The Trojan Horse should make a marvelous topic because the very idea of a monstrous wooden horse filled with warriors is fascinating. Helen of Troy is an intriguing figure because she is supposed to be the most beautiful woman in the world. You might, if permissible, write about the discovery of the ruins of Troy in 1871 by Heinrich Schliemann. You might look up about the "Judgment of Paris," which was what supposedly led to the Trojan War. If you can limit your paper to a single aspect of the Iliad, I think you would find your task much easier and more interesting.
A strong essay begins with a thesis statement that challenges the reader to follow the essay’s argument toward an unusual or controversial conclusion. So, if you began with “The Iliad is a story about the Trojan War,” you would not have a good thesis statement. But if you started with “The Iliad is outdated and no longer readable in modern times,” you would have stated a controversial (but, in my opinion, untenable) opinion. Look at the epic and find the most unusual thing about it that piqued your interest as you read it, and go from there. Were the characters psychologically convincing and consistent? Was Helen worth all those deaths? Who were the villains, if any? What part of the story did Homer not discuss, but which you wished he had? etc. So, the answer is: Find what interested you about the epic, and defend a controversial point of view.