I've pretty well watched both, so I may be of some help.
Essentially, there are no MAJOR differences between them, plotwise. It's a tough plot to deviate from. Where it really differs is in the portrayal of Gatsby. I think, personally, that he is slightly more mysterious in the novel. You almost see too much of him. In the novel, Nick (I think his name's Nick, it's been a year) is your only real gateway to Gatsby's character, and when you lose that as the only perspective, and have this visual input, it just kind of changes the way many characters, Gatsby in particular, are viewed.
But I don't think I have enough for you to write an essay on. I don't think you could without referencing specific moments of the movie in terms of the book and speaking on them. The murder is a good place to end. You see it, and in the book it's just kind of mentioned after the fact. This is a major difference in mediums that affects the way it is viewed. With these type of papers, you want to look at the characters and how they are portrayed, how the plot is followed, and the basic differences between books and films and how that difference affects this particular piece of literature in the adaptation.