From what I've heard from other students:
At the beginning of August, the bookstore will have a search feature to find out what books you will need for each class (or you can email the teacher)...From there you can either buy the books through your student account on the bookstore website and the books will be in your residence hall when you arrive...or you can order your books through other websites that sometimes have better prices (Amazon, Ecampus, etc.)
Perhaps someone else can offer some more advice.