If you're using Windows, you can just hit your print screen button. This saves whatever you see to the clipboard. Then open up Paint (or any other like program), hit paste and voila!
Other useful tips: Use the Crop function to only show what you want to see (e.g. not the open Pornhub tab on your taskbar). And when you go to Save, you can select Save As and change it to .jpg or .gif or whatever you need.