Create a list for each character and note key points about them as your reading reveals these points. The men's pasts, actions, motivations and involvement with the plot, themes and setting of the storyline are key factors.
Develop your own concepts about Tom and Gatsby so you can use explanation and analysis in your comparison. Do not merely cite facts. For example, you might say both have low moral values, since Tom breaks a woman's nose, and Gatsby has a criminal background -- then describe your ultimate findings on the topic for both.
Write a brief overview, or thesis statement, to offer a glimpse of what your paper will say about the characters and introduce your primary analysis. Adhere to any specific style information your instructor has provided. Otherwise, stick to generally accepted formatting for literary papers.
Write your observations comparing and contrasting Tom and Gatsby. You can address such concepts as the men's idea of love, the value they place on money or how they seem to approach the idea of the American Dream. You may write in blocks, describing Tom in entirety and then Gatsby, or include your observations of each character topic by topic.
Wrap up your thoughts and observations at the end of your paper, addressing the ideas raised in your opening statements. If you began by raising the question of who was more deserving of Daisy's love, you would address that question with your answer, and support it with why that is true. Provide the overall strongest impressions found in your analysis.