It’s easy…
Press the ‘Print Screen’ (PrtScn) key on your keyboard (hold down ‘Alt’ and press ‘PrtScn’ to capture only the active window).
Open Paint (or any other image editor), go to the ‘Edit’ menu and choose ‘Paste’.
Save the screen-shot (go to ‘File’ > ‘Menu’ > ‘Save As’, choice where you want to save the image and then type a file name for it).