How do I track my lost iPhone?

Did you enable Find My?

If you didn't turn on Find My [device] before your device was lost or stolen, it can't be used to locate your device. Change your Apple ID password. Change the passwords for other internet accounts on your device. This can include email accounts, Facebook, or Twitter etc...

Here you go --> If your iPhone, iPad, or iPod touch is lost or stolen - Apple ...

These are the steps you can perform in such a situation:--> If your iPhone, iPad, or iPod touch is lost or stolen - Apple Support

Regards

Giulio