How do I take a screenshot?
Just press the Print Screen key.
You may find the Pictures in your Home Picture folder, this will normally work with any operative system.
Shift + Print Screen allows you to print a selection of the screen.
Print Screen alone will take a screenshot.
Hold SUPER (The Windows Icon on you keyboard) to display all key shortcuts. (Only worked in Gnome 2.x)
View the available short-cuts under
Settings -> Keyboard -> Shortcuts -> Screenshots
or
Settings -> Devices -> Keyboard
If you have less options view Create custom short-cuts
If you would like to use a tool you may want to use Gimp, Shutter, etc. View more in This thread