How do I find my IP address on the iPhone?

It depends on which IP address you mean. An IP address is assigned by the network when your phone connects to it, so if you have a WiFi connection you can see the IP address in Settings/WiFi, then tap on the i next to the network name. Note that this will be the address on your local network, not the address that someone outside of your local network will see. For that use the next paragraph when connected to WiFi.

If you have a cellular connection it's a bit trickier. You need to go to a website that will return your IP address. One is Gibson Research, http://grc.com. Follow the links to the Shields Up! test.

Having answered the question you asked, is it what you meant to ask? Or is it some other value you need, like the phone's MAC address (WiFi address) or its IMEI (your cellular network identification)?