Know the characters - specifically Lennie, George, Curley, Curleys wife, Candy and Crooks. Know some of these in greater detail than others ( Lennie and George ) you may be asked to write about their relationship. Also know the intro very well. Know the important chapters and of course, the final chapters. Learn about the setting ( 1930s Depression , Salinas etc ) and Know the major themes such as Dreams and Loneliness. Other than that, make sure you can answer all the past exam questions on this topic and you'll be grand!