When writing an essay about Jason and Medea, there are many points of comparisons that you can make. I suggest that you pick one comparison and make it into your thesis. From this point, think of the way the mythology of Jason and Medea fits in.
For example, one of the most fruitful areas to talk about is gender. As you probably know, women were not seen as powerful at all in the Greek world. They were the subservient "other" in culture. So, to see Medea as a frightful and vengeful woman is something new. Some reflection on this would be helpful.
From this thesis, you can begin to explore the areas of Medea's strengths - magic. You might even make the point that Medea's power is "dark." You might even ask the question whether Medea has any heroic qualities. These thoughts should get you started.