If you're running Windows, in the top right corner there is a button labeled "Print Screen" click that button and then past it into paint or another image editing program.
If you're running OS X, press and hold Command+Shift+4, your cursor will turn into small cross hairs, drag a box over the area on your monitor where you'd like to capture an image. You can also use Command+Shift+3 to capture an image of your entire screen.