on your keyboard you have a button that says "Prt Sc SysRq" press that and you can open it up with windows paint or anyother program you choose.
LOL Geestar. How big is your keyboard? Outside of the Enter key on the right side of my keyboard, I don't have a key big enough to fit all of that on it. Mine says Print Screen, with Screen being below Print, on the button.
If you only want to capture the active window and not the entire desktop press ALT + Print Screen. The method Geestar20 said works fine but captures the entire desktop.
Now say what you want to capture is more than what is actually on your screen and you have to scroll to see it all, then Capture Wiz Pro( from www.pixelmetrics.com) is a great solution.
If you want to take screenshots from your web browser, Firefox has an extension available called screen grab! and it can take a screenshot of the webpage you're at, or the whole page, I've used it a lot since I got it. Here's the link if anyone wants it - https://addons.mozilla.org/extensions/moreinfo.php?id=1146