If you're wanting to crop a screenshot, it would probably be easier to do just take a screenshot of the area of the screen that you want in the first place. If you simply hit ⇧⌘4 (that's shift-command-4), you get little crosshairs that allow you to pick the region of the screen that you want. If you want to take it of just one window, press that combination first, hit space bar once, then click on the window that you want.

Also, Preview is the built-in photo viewer; it should be in your Applications folder. To crop an image in it, just select the region that you want to crop with the select tool (Tools→Select Tool or ⌘3) and then use Tools→Crop (or ⌘K) .