Do you want to take a picture of your screen?
Press the Print Screen or Prt Scr button.
Your screenshot will not be saved as a file, it is copied to your clipboard. You can paste it into any program or even into an email as a normal image by pressing the "ctrl + v" keys.
Active Window Screenshots
If you don't want to take a picture of the entire screen, press the Alt key and Print Screen key simultaneously. You will then get an image of the active window only.
Alt + Print Screen.