You have the actual ubuntu, or do you have kubuntu? Anyway, in ubuntu (with gnome), you should be able to make a screenshot with your good old print screen key. If not, you could check your keybindings (typically found in the System->Preferences->Keyboard Shortcuts menu.
If that doesn't quite work out, you can always use the screen capture feature of the gimp, which (I'm pretty sure) should come with a fresh install of ubuntu (Applications->Graphics->GIMP Image Editor)