If you are using a PC then you should have a key on your keyboard marked "Print Screen". It might be one of the "F" keys or another key along the top row. You might have to press and hold another key to access the print screen function. For example on my keyboard I have to press and hold the "fn" key and then press the Home key.
"Print Screen" will take a snapshot of the entire screen. If you want a snapshot of just one application window then click on that window (to make it the active window) and press and hold the "Alt" key and then press the "Print Screen" key.
Once you have captured your screen or application, bring up Paint and paste the image (Paste will be on the Edit menu, or you can just press and hold the Ctrl key and press the "v' key).