You can do either of the following things:

Press that button on your keyboad 
labeled "PrtScn" (also has "SysRq" written on 
it), located in the bar of three buttons directly 
to the right of the F1-F12 keys.

If you press this button (sometimes a little 
window will pop up asking if you wish to copy 
your screen. Click "Do not show again" and pres 
the button again, as what usually happens to me 
is that the warning window is copied as well 
^^;;), the whole screen will be copied and placed 
on your clipboard. Then you can open your 
grpahics program of choice, paste the picture, ad 
cut what you like out.

OR:

Press Alt+PrtScn, and it copies the active 
window. This just makes it easier during the 
editing process, really.

I know that sounds complicated ^^;; But it's 
really not.

Basically:

Press PrtScn. Open grphics program. Paste.

OR

Press Alt+PrtScn. Open graphics program. Paste.

Hope that helps ^^

-Misha