how to find my i phone

1. If your iPhone, iPad, or iPod touch is lost or stolen - Apple ...
2. Find the serial number or IMEI on your iPhone, iPad, or iPod ...

You may use a web browser to get Serial Number and IMEI Number. You cannot track using iMEI number. but you may need it so keep it handy

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...