Speak to Priestess Josetta in Elwynn Forest.
Hello again, young priest. The time has come for you to learn more about our religion. After you've completed whatever tasks you have here in Northshire, please seek out Priestess Josetta in Goldshire. She has more to teach you and has heard that you are growing strong in the ways of the Light. She looks forward to tutoring you more.
I'm glad you've arrived, <name>. We have much to discuss about your future and your path within the Light.