Thank you very much for your patience!
If you have iCloud or Find My iPhone activated on it then you can track it. On your PC or another device, go to www.icloud.com/find
Then log in with your Apple ID or the iCloud account you used on your iPhone. Click the 'Find My iPhone' button. Then click All Devices, then select your iPhone. This will show the iPhone on the map. If you are unable to track it, then click Lost Mode there. This will password ***** it and show the custom message you want on it.
If iCloud still does not find it, then click Erase there. This will erase the iPhone and deactivate it. No one will ever be able to use that iPhone without you activating it again.
(Once it is erased, you need to provide your Apple ID and password ***** activate it again)
If you have not enabled the iCloud and Find My iPhone option on it, then I'm sorry, you will not be able to track it. It can't also be tracked if it is turned off. But it will be erased the moment it turns on (assuming it is connected to the internet). Find my iPhone option is your only hope now.
If your iPhone isn't locked with a passcode or Touch/FaceID, then I'd also recommend changing your email password, AppStore password, and all other passwords you used on it. So the person who gets the iPhone will not be able to access your emails or other things that require a password. Let me know if you need help with this.
If you have a passcode or Touch/FaceID enabled, then you don't have to change any passwords as the person who gets it will not be able to access anything. It will disable itself if there are any unauthorized attempts to open it.
You should also call your carrier and ask them to disable the SIM and services as well as backlist the iPhone.
If you have any questions or would like any additional information, please don't hesitate to ask.