In order to activate USB Debugging you need to navigate to the Settings of the device.
From Settings you need to activate first the Developer Options menu which by default should be hidden.
Developer Options can be activated by tapping 7 times on the Build Number which depending on the device manufacturer can be found in:
Settings -> About Phone or About Device -> Build Number
Settings -> About Phone or About Device -> Software Information -> Build Number
After you activated Developer Options go back to the main Settings menu find and access this menu then put a check mark on USB Debugging.