Lost 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

Don'ts

If you remove the device from your account after you erase it, Activation Lock will be turned off. This allows another person to turn on and use your device.

Do's

• Mark As Lost.
• Erase your device.
• Apple Care+ :- file a claim for your lost or stolen iPhone. (If you had Find My enabled)

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. 

The best would be to Change your Apple ID password. Change the passwords for other internet accounts on your device. This can include email accounts, Facebook, or Twitter etc...