Hi and Welcome. My Name is ***** ***** I look forward to assisting you today with your question.
I'm sorry that you lost your iPhone. If you have the internet and iCloud or Find My iPhone feature activated on it then you can track/find it. On your PC, go to www.icloud.com
Then login with your Apple ID or iCloud account you used in your iPhone. Click Find My iPhone button. Then click All Devices, then select your iPhone. This will show the iPhone in the map. If you are unable to track it, then click Lost Mode in there. This will password ***** it and show custom message you want on it.
If it still not find it, then click Erase there. This will erase the iPhone and deactivate it. No one will ever able to use that iPhone without you activate 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 Fine My iPhone option on it, then you will not be able to track it unfortunately. It can't also be tracked if it is turned off. But it will be erased the moment it turn on (assuming it is connected to internet). Find my iPhone option is your only hope now.
I'd also recommend to change your email password, AppStore password ***** all other passwords you used on it. So the person who get the iPhone will not be able access your emails or other things that require password. This is not necessary if it is already locked with passcode.
You should also call your carrier and ask them to disable the services as well as backlist the iPhone.
If you've any questions or would like any additional information, please don't hesitate to ask.