Enable the USB Debugging option under Settings > Developer options.
For Android 4.2 and newer, Developer options is hidden by default; use the following steps:
- On the device, go to Settings > About <device>.
- Tap the Build number seven times to make Settings > Developer options available.
- Then enable the USB Debugging option.
- Tip: You might also want to enable the Stay awake option, to prevent your Android device from sleeping while plugged into the USB port.