how to lock

See information in If your iPhone, iPad, or iPod touch is lost or stolen - Apple Support

including the following:

Mark As Lost. This will remotely lock your device with a passcode and you can display a custom message with your phone number on your missing device's Lock screen. It will also keep track of your device's location. If you added credit, debit, or prepaid cards to Apple Pay, the ability to make payments using Apple Pay on the device is suspended when you put your device in Lost Mode.