Where's the IP address shown on an iPhone?

If the address there starts with 169.254, the phone hasn't actually got an address from the router. Forget the network and rejoin it, or restart the router.

Settings > Wi-Fi, tap the little (i) next to the network you're on, and it's listed as IP Address. That's your local address on that network though. Your public IP (what websites see) is different and you'd need to search "what is my ip" to see it. Don't confuse it with the Private Wi-Fi Address line either, that's a MAC address.