Backpacks with just one big pocket mean that everything gets lumped in together.

Choose one with several pockets to organize your stuff better. Then the weight will be evened out, which will be better for you to carry.
A waist strap keeps the bag from sliding about and it also takes some of the weight so that your shoulders don't have to do all of the work by themselves.

If you do have a lot to carry and there are no lockers at your school, then it may be a good idea to get a backpack that has wheels and a pullout handle.

First, check with your school to see that it's okay to use this type.
It can stand next to or sit under your desk or your seat so that other people are safe.
It also means that you only have to pick it up to put it on the bus or train or into the car – the rest of the time you can just pull it along.
Still, watch where you and the backpack are going in case you run someone over with it!
Of course, these kinds of backpacks will cost more, but they should last for years, and so will your back!