To easily secure your device enable a lock code to unlock it. Follow these steps to learn how to enable a lock code.
From the home screen, swipe up to open the main menu.
Tap Settings.
Tap Fingerprint, Face & Passcode..
Tap Lock Screen Passcode.
Tap Turn on Passcode.
Enter a lock code.
Re-enter the lock code and tap OK.
Enter your email address, so you can reset your passcode.
Now that you have set a lock code, you will need to enter this code everytime you are unlocking your device.