Depending on your comp, hit "print scr" or ALT "print scr"
OPen (or previosly open) MSpaint. Go to edit, and hit "paste" the screen shot should paste into MS paint. Crop it, edit it or whatever, save it then host it wherever(imageshack.us or photobucket.com for example-I prefer photobucket).
It's free and awesome. I've configured it so that when I hit PrintScreen, I can drag a rectangle over the portion of the screen I want captured, and it saves a PNG file right on my desktop. It couldn't be simpler!