I need to shut down my iPhone it has been stolen

To locate it in the Find My [Device] app, you must have activated the Find My app BEFORE the iPhone X was lost. Here is some information on that app ➔ If your iPhone, iPad, or iPod touch is lost or stolen - Apple Support.

Note that your lost iPhone must be able to make the connection to the internet via Wi-Fi or cellular connection so that it can mark its location and broadcast it. 

Axel F.

Hey there!

Your phone cannot be powered down remotely. You can see information about Find My at Find My - Official Apple Support