No one else, of course, can know your opinions after reading the Iliad but it is possible to offer some guidance in determining what to write. I would do the following.
Since there is so much going on in the Iliad, I would first jot down from memory what stuck out for me. This would be in the form of brainstorming, so I would write down anything, no matter how silly it seemed. For me, the list would be like something like: I love when Hector and Andromache are saying goodbye to each other and their young son cries because he is afraid of his father in his armor with the waving plume on his head. I wish Hector didn't have to go to war. What about those gods and goddesses often being right next to the humans but invisible? It's weird from a modern perspective. Why is Achilles such a baby about fighting? Why do these people have to fight—and why do the gods want to drag out the war so long? I love the new shield of Achilles for depicting all the peaceful scenes the soldiers are fighting to protect. Why are the women treated like war booty? Why is there no empathy for Briseis being passed around from man to man? Why does Patroclus have to get killed?
After looking at what I had jotted down I would note themes. My notes above show I am developing the opinion that the Iliad is telling me that war is a tragic business. That could become the basis of a thesis statement. The important thing at this point is to find quotes or examples to back up what you have to say. For example, if your opinion is that the Iliad shows war is a tragic business, back it up with Patroclus's death, Achilles's grief over the loss of his friend, women being snatched as war booty and raped, and other examples of violence. Your opinion always has to be supported.
You no doubt understand that there is little more irritating than saying "my opinion is that the Iliad bored me" or "I couldn't find it relatable." Focus on where you weren't bored or could relate, and work from there.