how can I report my iPhone was stolen?

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

As edclange has mentioned, Apple does not directly get involved with lost or stolen iPhones. Instead, they provide you with a few options that are described in the support article link I just provided you.

See Apple's suggestions here:

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

Contact your cellular carrier and your local law enforcement. Apple does not get involved.