Press the PRINT SCREEN Button (PRTSC or PRTSCN on some keyboards).
If your keyboard looks like the one above you should press Shift+Prtscn
Some (laptops) might use other keyboard combination, such as FN+INSERT, or FN+PrtScn. (FN key is usually in the lower left near the space bar)
To copy only an active window
Press ALT+PRINT SCREEN.
Now you can then paste the screen capture into a document.