QUICKLY!

Easy!

Ты hit the "Print Screen" button on the вверх right side of your keyboard.
It's to the left of "Scroll Lock" and "Pause Break"

That is basically like right-click=copying your whole screen, so Ты have to paste your screencap somewhere and save it.

MS Paint works really good for that. Then Ты can do what Ты like with it, like save it, Редактировать it, etc. или save it and Загрузить it to some other program.

Hope this helps!

Either use prntscrn button on your computer, the snipping tool if Ты have Windows или use VLC player to screencap a video (there's a really good tutorial on this somewhere in livejournal).