Photo Courtesy of CNET
"After misplacing my phone for roughly 24 hours and then happily reuniting with it, I have learned a few lessons about securing an iPhone. These tips will not only help keep your data safe during the uneasy time you and your iPhone are separated, but they will also help you track down your lost iPhone. Let's get to it.
1. Use a passcode
First step: set up a passcode. Do not walk around with an iPhone that anyone can swipe to open. Protect your iPhone with a passcode -- preferably a six-digit passcode or a custom numeric or alphanumeric code. With either Touch ID or Face ID, you won't need to enter your passcode unless your finger is wet or you are bundled up to a degree that your face is obscured. You can set up a passcode by going to Settings > Touch ID & Passcode or Settings > Face ID & Passcode and enroll your fingerprint or face. Make sure iPhone Unlock is toggled on after you set up your passcode."
Read more about iPhone security at CNET