You can set your phone to use your fingerprint as phone lock code or to authorise iTunes and App Store purchases. To use Touch ID, you need to turn on the phone lock code.
Press Touch ID & Passcode.
Press Add a Fingerprint... and follow the instructions on the screen to add your fingerprint.
Press Continue. If you haven't previously selected a phone lock code, key in a code of your own choice twice.
Press the indicators next to the required settings to turn them on or off.
Press the Home key to return to the home screen.