QUICKLY!

Easy!

あなた hit the "Print Screen" button on the 上, ページのトップへ 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 あなた have to paste your screencap somewhere and save it.

MS Paint works really good for that. Then あなた can do what あなた like with it, like save it, 編集 it, etc. または save it and アップロード it to some other program.

Hope this helps!

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