just press print screen (on my keyboard it says "PrtScn", probably the same on yours), and then open up whatever graphics software you use (ie Paint, Gimp, Photoshop etc), and press "Paste", and then save it (prefereably as a jpeg)
But if you plan on taking more pictures then one you are best to get xfire or fraps (only two that i have used) and they have a hot key which u can press as many times you want and when you get back to desk top they are all in a folder I use Fraps now because my xfire seemed to not work after i reformated
my favourite is a little app called Grabby, i'll attach it in a min, it works great and has tiny resource usage, just a shame it was last updated in 2001!