Pick any image and crop it into a 4X4, 4X5 and 4X6 format. My DSLR naturally has a 4X6 format while many Point and Shoot cameras use a 4X5 format. Study the impact the image format has on the subject of your composition.
I started out with a simple flower shot. The feeling of the subject is totally different in each format I used.
Enjoy and happy photographing!