It should be enough to simply use the Print screen key to get a window with the full screenshot and options to save it. But using an application with more options is nicer in most cases. There one screenshot application for gnome (gnome-screenshot) usually found under Accessories menu, and one for kde (Ksnapshot) usually found under Graphics menu.

If you need to edit screenshots after taking them, like adding highlights or text Shutter is an excellent choice.

All of those applications can be installed the usual way, they're in repositories.