If you have a FN button (function) hold that and press the Prnt Scrn button. Or just press the Prnt Scrn button. Minecraft you can do so with F2.
press the prnt scr,prnt scrn, prntscn, prnscrn, or prnscn button. if you are running fraps, there is a screenshot hotkey in the options.
using fraps, it puts them in whatever folder you have it set to send them to, mine get sent to a folder called frapscreenies on my desktop.
then when you press the prntscrn button, it puts it in your clipboard, you have to enter paint or photoshop or some similar program, and edit>paste it into a new image