1. To take a picture of your entire screen as it appears hold down the Command+Shift+3 keys in that order.
You will hear a snapshot noise and the image thumbnail will appear on your desktop.
2. To take a picture of a selection of your screen, hold down Command+Shift+4 in that order.
Use the tool to select the area in which you want to tae the picture of.
3. To take a picture of a window on your screen, hold Command+Shift+4 in that order, release all three keys and tap the Space Bar. Then click the window which you want to take the picture of.