There are a few simple steps you can take to composing a well-written essay comparing these two films.
First, make a list of the major similarities of each film. At this point make a long list of ideas. Be as general and as specific as possible. Often one idea will lead to a better idea. Don't worry about listing the differences at this point. I find that it is easier to note differences based on similarities. Otherwise, you may come up with a list of differences that have no apparent relevance to the rest of the essay.
Once you've created a long list of similarities (10-25 items at least), determine the main focus of your essay. You can do this in a number of ways. You can think of categories like setting, characters, themes, plot details, conflicts, etc.
It is possible that you could base your entire essay around one theme or conflict the movies both seem to share and the three ways each presents it differently. Or, you could look at three techniques from the above list and write about how the two movies are similar and different in each category. Just remember that creating three categories for your ideas will be the three avenues by which you will compare and contrast these two films (and the topics for your body paragraphs).
Organize your ideas into an outline to make sure each paragraph has a clear focus of its own, but is also focused on one overall topic (your thesis). It is much easier to revise an outline (by adding or subtracting information) than to re-write an entire rough draft. For further information, consult the link below.