Speak to Dark Cleric Beryl in Tirisfal Glades.
There are many lessons that the Forsaken have learned through experience--things that some races can only begin to fathom. And although our path lies in darkness, you will find that a great many things rely on both the light and the dark. Your lessons shall teach you to use both... and with discretion. But heed my words: never forget we were forged in corruption and left to rot like so much carrion. The Darkness is our home.
Seek out Dark Cleric Beryl in Brill when you're ready to learn more.
Finally, you've arrived. Your faith alone will not allow you to pass the trials before you, young one. A sense of urgency and some ambition will help too.