Photos at Kmart etc are are cropped to fit the paper. Photo paper for small prints are generally 4 x 6, so if your photo does not fit this aspect ratio, it gets cropped.

In photoshop, once you select the crop tool, and it appears just under the menu, you can click on the tool in the menu bar (not in the tools palette) and choose an aspect ratio that you want to crop to, then do your crop. It is a matter of cropping to the same aspect ratio as the print will be.

So no use cropping to 4 x 8, if you are printing on 4 x 6 paper, otherwise your lose some of the image.