If you're on a PC, taking a screenshot is simple enough. The cool thing is you can do it with whatever is displayed on the GUI. Whether it's the desktop, windows you have open, websites, popups, anything.

Simply hold down the CTRL key then press "Print Scrn" (it's to the upper right of your keyboard, right of the F keys).

CTRL + PRINT SCRN will capture the entire screen to your clipboard.

CTRL + ALT + PRINT SCRN will capture the upper most window. So if it's only what's in the browser window or something, for example, that's what it'll capture and not anything that's "behind" it.

Then if you use Photoshop or any other photo editing program, just create a new file and PASTE the screengrab in to it and there ya go!