Of Mice and Men is one of the best piece of art that I have ever read. Steinbeck's use of language is just amazing, very admirable. I think your best bet is to read it again between the lines and memorize the pages of good quotations! This will come really handy during the exams (N.B. I said memorise coz you'll be given new books during the exams, this is for my school's case only - not sure with others).
The film was good but book>film.. though it would be good if you read the book first, then watch the film.. rather than the other way round (coz if you watch the film, you might skip some of the pages - not good idea - as you have to have a clear idea of how the picture fits).
Ask your teacher to go through it again in class. I am pretty sure that you should be practising writing essays on this right now. Because if you are not, give the teacher a piece of your mind. Just kidding. You need to practice writing essays. Just try to write essays and use mindmaps to plan it. This will really help you prepare for the exam. (don't time yourself yet, for the first few essays..) I know that this just sounds awful, but trust me once you're into it/in the zone... you'll enjoy it and will want to write some more.
Good luck.