Even though this novel is not long, it does have many topics to choose from for writing an essay. You might consider some of the themes, such as friendship or loneliness. During the period of time that the novel takes place, the Depression, it is very unusual for two men to have a close friendship like the one George and Lennie have. The boss on the ranch thinks George might be taking advantage of Lennie because Lennie is mentally disabled. Even Slim is surprised about their friendship. Most of the men are alone, travelling from one place to another to find work.
Another possible thesis for your essay would be George's decision to kill Lennie rather than allow him to be captured and sent to prison. Most people feel that George's murder of Lennie is a merciful act. I have found that my students find this topic interesting to write about, and you usually do a better job if you like what you're writing about.
I hope this helps. Please don't hesitate to ask a particular question if you need to. The links below will help you further.
There are many possible themes and ideas you could address in a paper about Of Mice and Men. Here are some ideas you might want to consider-
Consider the themes listed in the enotes summary linked below. How are these themes expresed through the characters in the novel?
You could also research an aspect of history related to Of Mice and Men. Think about the great depression and how it affected employment. There are plenty of ways this is shown throughout the novel as well.
I hope this gave you a place to start with your thesis brainstorming.