Finding your IP address on a Yealink T19P is very easy.  

With the phone displaying the Idle screen (this is the normal standby screen which shows the time, date and the four soft keys) press  which can be found in the middle of the directional arrows. 

The IP address should then be displayed on the first line. 

Please note that if the phone has no IP address, it is not connected to your local network connection correctly and you will need to check your local internet and also that the ethernet cable is plugged in correctly.