Use the button "Pr Scrn" on keyboard, without minimizing what u wanna capture then press it. U should goto graphic editors like MS Paint and Press CTRL+V, save the image…
U could also try PCHand Screen Capture. It could help u capture anything on the screen, it offers seven capture modes. It also provides a build-in editor with which u can edit and save the captured images. Im using it, it is easy to use, even for a Novice. …Hope it could help u!
http://www.screen-capture-record.com/