how do i locate a stolen phone

You can view this helpful document for more info: iCloud: Locate your device

Essentially you log into iCloud.com with your Apple ID and see if the device is still online or powered on. You can then remotely erase, lock, or send a message to appear on the screen of the device. If it has not been online in some hours it's last known location will appear until it has been offline for a significant number of hours. If it is offline, there is nothing you can do to change the situation.