On the top left of your key board theirs a button that says Print Scrn....... right next to Scroll and F12. Then just push that and it takes the picture of your desktop.

Now go to Paint and hit the edit button and scroll down to paste, hit paste and that will paste it to paint. Now all you have to do is save it.

Does that make better sense to you? Now I have windows so if you happen to have mac, I don't know if it works the same.