Size. When you buy a backpack, get a very large one because you will have many large books to put in it.
Laptop. Be sure it has a protective area for a laptop if you have a laptop computer.
Comfort. Get one with padded straps, and verify that it feels comfortable on you, because it may be on your back hundreds of hours by the time you graduate.
Rolling Suitcase. Some people buy a small rolling suit case so they don’t have so much weight on their back all day. However, this can be heavy to carry up stairs, difficult to pull through a crowd, and some would say it’s nerdy.