Click on the window you want to take a screenshot of.
To take a screenshot of the entire screen, press the Print Screen (it could also be labeled as PrtScn or PrtScrn) button on your keyboard. If you want to take a screenshot of a specific window, you will need to press the ALT and Print Screen button at the same time.
After the PrtScn is done, you it is now ready to be pasted into any document.