how do i get back my stolen device?

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

You need to have turned on Find My iPhone from this point on. See the steps in the first link below especially, but don't ignore the second.

If your iPhone, iPad, or iPod touch is lost or stolen  https://support.apple.com/en-us/HT201472

Use Lost Mode in Find My iPhone on iCloud.com https://support.apple.com/guide/icloud/use-lost-mode-mmfc0f0165/icloud

If Find My iPhone is offline or not working https://support.apple.com/en-us/HT204233