In windows:
Hold down ALT, press PrtScrn
Open Paint app, hold down CTRL and press V
Resize/cut/whatever/Save.
In windows also:
Hold down Windows Key and press PrtScrn. Wh0le screenshot will be saved somewhere in your Pictures folder (Screenshots).
On iPhone:
Press Sleep button and Volume Up buttons together at the same time.
On Android phones:
Depends on how old the software is and possibly who made the device.
On a Mac:
Press Shift + Command + 3