Apple offers a number of protections for iPhones and iPads that have gone AWOL. Your PIN and Touch ID won't protect your device but will safeguard the data on it. You can (and should) also disable Siri on the lock screen so that the voice assistant doesn't disclose any info.
If you haven't locked down your phone with one or more of those options, your best bet for a lost or stolen iPhone is to wipe it remotely. To do so, you must have previously set up Find My iPhone. Tap the Settings app and iCloud, toggle Find My iPhone to on, confirm your password in the pop-up, and finally tap Turn On.
If you have Find My iPhone enabled, head to icloud.com/#find and enter your Apple ID and password. Once the webpage launches, click All Devices, select your missing device in the drop-down menu, and choose Erase iPhone.