Speak with Matron Arena - the Priest class trainer - inside the Sunspire on Sunstrider Isle.
As you advance within your class, you will want to take advantage of the training that your class trainer can provide you. For young priests here on Sunstrider Isle, your trainer is Matron Arena. Speak with the matron and see what training she has available for you. The things she will teach do have a cost, so bring some coin with you.
Matron Arena - as with all the trainers on Sunstrider Isle - is inside the Sunspire on the lower level.
Welcome, <name> - I am Matron Arena. I offer my services to those who seek to learn the ways of the <class>. Such a path is often misconstrued as a path of pacifism; for we blood elves, this couldn't be further from the truth. While we do mend both bone and spirit, we also act as a fist of vengeance for those who would transgress against us.
I will train you, but all I ask is that you cover any training costs that may come up, and that you learn with an open mind. If you agree to this, we may begin.