On the keyboard, press "Print Screen" or "PRT SCR" button.
Open Paint, press Ctrl+V to paste it.
On the upper menu, save it and there's your screenshot.
There's a super easy way to do this, download a program called "Dropbox". It's free and it takes a screenshot instantly and saves it whenever you press "Print Screen". Then there are also programs like ScreenHunter but their free version saves in normal quality JPG format.