VirtualKid, if you hold down the following 3 keys: Apple, Shift and 3 then the Mac will take a screenshot of your entire desktop.

If you hold down Apple, Shift and 4 then a "target" type of pointer will show. Place this pointer at the upper-left of what you want to take a picture of, hold down the mouse button and drag the mouse to the bottom-right of the image. When you let go of your mouse button you will have your new image.