On your iPhone, go to Settings> [yourname]> Find My.
If you’re asked to sign in, enter your Apple ID. If you don’t have one, tap “Don’t have an AppleID or forgot it?” then follow the instructions.
Tap Find My iPhone, then turn on Find My iPhone.
You can also turn on or off either of the following:
Find My network: If your device is offline (not connected to Wi-Fi or cellular), Find My can locate it using the Find My network.
On a supported iPhone, turning on the Find My network allows you to locate your iPhone for up to 24 hours after it’s turned off, or up to 5 hours when it’s in power reserve mode.
Send Last Location: If your device’s battery charge level becomes critically low, its location is sent to Apple automatically.
Your iPhone also includes a feature called Activation Lock that prevents anyone else from activating and using your device, even if it’s completely erased. See the Apple Support article Activation Lock for iPhone, iPad, and iPodtouch.