How to block my iphone

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

If you want to lock it, you first need another device (except. If you have one, just go to Find My on that device, go to Devices > Your iPhone. Click on it, and in the prompt you'll find "Mark as Lost". Below that it says "Activate". Press that, go through it, then press "Continue", and then it will be locked!