You can set your mobile phone to use your fingerprint as phone lock code or to authorise iTunes or App Store purchases. It's easy, but remember you need to turn on use of phone lock code before you can use this function.
Tap Touch ID & Passcode.
Tap Add a Fingerprint.... Follow the instructions on the display to add a fingerprint.
Tap Continue. Key in a phone lock code of your own choice. Key in the same phone lock code again.
To use Touch ID as phone lock code: Tap the indicator next to "iPhone Unlock" until the function is turned on.
To use Touch ID to authorise iTunes or App Store purchases: Tap the indicator next to "iTunes & App Store" until the function is turned on.