To take a screen shot simply press the Print Screen(Prt Scr) key on your keyboard -usually on the upper right next to the f12 key.
If you play in a window(small screen) hold the Alt key & press Print Screen key to capture the active window (last window clicked by mouse).
then..open paint or any graphics program you have...got to file/new, when the graphics program creates a window..hit paste(ctrl-v).
Make sure you save the file as filename.jpg.
You can upload it to places like
http://imageshack.us/