Choose one that is sturdy and will fit all their notebooks, binders and supplies. Make sure you know what type of backpacks your child's school allows. Some schools do not allow bags with wheels, some require them to be clear, while other schools only allow drawstring bags.
My kids' middle school only allow drawstring bags carried around the school and the one pictured above was the sturdiest one I've found. We've tried many different ones over the years and this hasn't ripped or tore after using it daily for 6 months (so far). Plus, it was $7 at Walmart.
If your child's school doesn't require one, I recommend you purchase a student planner. In it your child should record all his homework as it's assigned. Each night check their planner to insure all their homework and any projects are completed. This is also a wonderful place to leave messages for their teacher and for them to respond back.
All my kids' schools require planner similar to the ones sold here. Ours are purchased directly from the school for $5 each.
This works great for elementary schoolers, but it is used in higher grades as well. Basically, you designate one folder for all homework. That way your child only has to check one place for all their homework - not individual binders/folders.
I highly recommend using a 2 pocket poly folder like the one above. They last a whole lot longer than the paper ones that rip pretty quick with daily use. They even come in a variety of colors and only cost $1 or less.
This one is obvious - keeping a supply of pencils on hand. I try to make sure each of my kids has at least 3 pencils in their backpack each morning. There really is no excuse to lose 3 pencils.
I also want to suggest skipping the pretty designed pencils because they usually are harder to sharpen. After working with many teachers over the year most recommend using Dixon Ticonderoga brand pencils because they are easy to sharpen and good quality. I realize they do cost a bit more, but I've found them to be worth the cost.
These are obviously the basics and there are plenty more supplies that would help your child be even more prepared for school. These are the brands/products that have worked for our family for quite a while and I know they can help your family too.