Microsoft Windows
On your keyboard press the "Print Screen" button. This will make a copy of your screen and save it as an image to your clipboard.
Next open MS Paint and paste (Ctrl + V) this image from your clipboard to Paint's canvas. Finally, save this image as a png file : File > Save As ... Select png under 'file type' and save to your desktop.
Apple Mac OS X
On the keyboard press the following three keys simultaneously: Command + Shift + 3.
This will create a .png image of your screen and place it on your desktop.
