No matter who you are, if you want carry your things more relaxedly, you could choose backpack. Carrying things on your back is an ancient tradition, proven successful and useful time and time again. Carrying a backpack is a healthy and safe way to carry a load without hurting yourself. A single strapped bag or a purse can strain your back from the uneven distribution.
A backpack should not weight more than 15% of your body weight or 20 lbs, whichever is lighter.
A bag with wide straps will distribute weight more evenly and won'te hurt.
A padded back will help to keep items inside from poking into you.
Look out for bags made of lighe materials. If a bag is heavy without anything in it, then imagine how heavy it will be with all of your books!
Get a bag with two straps and a waist clip for the most even distribution.
Make sure that the zippers are sturdy and won't break.
Sooner or later, it's going to rain. Try and get a waterproof and tearproof bag.