First, you'll need either the ISBN Number (located right above the bar code on the back of the book usually, write it down) for each textbook you'll need for class or the title, author and edition number for the textbooks.
Next, it's best to get the prices from the local student bookstore or even buy the textbooks you need (check the return policy of course). You don't want to have classes start and not have a textbook to use so while you're conducting your money saving search, you should have something handy. Of course, if classes haven't started yet, you're in even better shape!
Now, time to do some detective work to find those cheap textbooks! The sooner you start your search, the better. Check www.half.com and www.amazon.com by searching for either the ISBN number or the author and book title. Check the used book section for sellers selling their used copies of textbooks. Always make sure to figure in shipping distance and price of shipping. Unless you order expedited shipping, your textbook could take up to 10 days to arrive (it will usually come within a few days as long as you select a seller that isn't too far away from you). Make sure to read the condition of the textbook and whether it has any markings in it, you want to know what you're getting.
Now, if you've previously purchased your textbooks, make sure to return them before the return date, you don't want two copies of the same book! If prices are still too high, you can also try buying a different edition of the textbook. Usually the differences in editions are minimal. Good luck finding cheap textbooks! See my other guides for making money off of your used textbooks!