Use Find My on another Apple device or at iCloud.com/find
Sign in with your Apple ID
Select the stolen iPhone from your devices list
Turn on Lost Mode
Set a passcode if prompted
Add a message and contact number for the lock screen
Keep the device marked as lost
If needed, erase the iPhone remotely
Change your Apple ID password immediately
Contact your carrier to suspend the SIM or service
Report the theft to local law enforcement with the serial number or IMEI