I would get a few theory books (intro-y kinds). Amazon will have some options, but I always find that the more time I have with theory the better. I did this before I started my MA program in Anthropology in 2014 and I plan on doing the same this summer before starting a PHD in Sociology. You'll have to read the theorists actual work when you start the program, but this will give you the context that those who majored in the social sciences have when they start the program with you. As the person above said, however, a lot of people will have backgrounds outside the field.