How to find lost iPhone?

Start here:

Locate a device in Find My on iPhone - Apple Support

Locate a device in Find My on iPad - Apple Support

Here is Apple’s advice for lost or stolen devices:

If your iPhone, iPad or iPod touch is lost or stolen – Apple Support

Use the Find My app to locate a lost or stolen device – Apple Support

If the Find My iPad feature was enabled prior to it being “misplaced”, then if active, you would stand a good chance of locating it. 

However, if the feature was not activated prior to being misplaced - or the iPhone has been inactive for more than 24 hours, the last known location will not be available - and the opportunity to use technical means to recover the iPhone may have been lost.

Ensure that you Inform your Cellular Carrier of the theft - as necessary steps must be taken by them to safeguard your Cellular account from charges associated with fraudulent use.

You may wish to start with this link:

If your iPhone, iPad, or iPod touch is lost or stolen - Apple Support