If you're on Windows, look for the "Print Screen" or "PrtScr" (or something similar) key no your keyboard, press it and that might bring up a "Sniping tool" (I think) window, using that, you should be able to select an area of the screen to capture and save to a file.
If the sniping tool doesn't come up by itself look for it in the start menu under accessories (I think...).
Third option is you hit Print Screen then open up Paint and then hit Ctrl+V or paste. That should show the entire screen capture, vhish you can then trim to size and save to file.
I'm telling you this of the top of my head because I stopped using Windows, so some things might not be fully accurate, but it should be close enough.