This is a two part question. First, you need to know what MLA formatting is. The quickest way to get acquainted with MLA formatting is by looking at a website that shows you the basic point of MLA formatting. I will add a link. This website will cover topics from font size, margins, citations, and much more.
Second and more importantly, you need to write a research paper. This means that you need to do some research on your topic. You need sources, probably the more, the better. Since Of Mice and Men is a classic, there will be many scholars who have written about the topic. A good way to go about conducting research is to go to your school's library and see if they subscribe to electronic journals such as Project Muse or JSTOR. These services have hundreds of journals and you can do key word searches. After this, pick the articles that you would like to use and you begin your paper.
The process is hard at first, but it gets easier.