A great way to increase security on your device is to lock individual apps that contains sensitive or private information. You can use the procedure below to lock any apps including contacts, SMS, e-mail, gallery, settings, and calls.
You can set apps to be locked with a password, or pattern lock screen.
To turn on individual app locking, go to:
Setting –> Additional settings –> Privacy –> Privacy settings –> turn on Lock individual apps
As you can see from the pictures above.