Easiest & Best way to make a box shelf
Go to the hardware store and pick up a miter box and; if it doesn't come with one, a back saw.  While your there pick up a set of chisels, that's really all you need to make the shelves.

Here is a good description of how to cut the rabbet by hand using a back saw. 

Warning: You may want to practice on some scrap wood until you feel comfortable.

If you don't mind spending the money you could buy a router and a rabbet bit, this would be the fast more efficient way to make the shelves.