Forget about the f12 Cent0, just use the print screen button as others have stated. It does not matter what version of windows you use either. Paint is a program that comes with all windows operating systems.
To find paint do the following. Click on the start bar in the lower left of your toolbar at the bottom of your desktop. Click on all programs, Find the folder named accessories and click on that, then find the paint program. Once the paint program is opened up, hold down the control key (Ctrl) found on the lower left of the keyboard. While you are holding down control, press V. This should insert the image or "screenshot" into Paint. Once that is done click on file, navigate to "save as" and save the image to an area on your desktop.