I'm not sure how to do it on a Mac computer, but if you're using a PC running Windows here are the steps to follow:
(1) Press the PrtSc button. (It may be labeled "Prt Scrn" or "Print Screen") It's normally near the upper right part of your keyboard. You won't see anything happen yet.
(2) Launch the Paint program. Usually you can find it in the Accessories folder in your Start menu.
(3) In Paint, click Edit, then click Paste. (Or, while holding down the Ctrl button, press the V button.)