Report to Gakin the Darkbinder in the Mage Quarter of Stormwind.
Hmph. Times past, all warlocks in Khaz Modan were trained under the auspices of the masters of Gnomeregan, but now that we've all been forced to relocate into this rat's hole in Ironforge, and Magni's decided to keep a careful watch on us, we're forced to send our novices to Stormwind for training.
Luckily, Gakin's no slouch, so you'll still learn what you need to know, but it burns my blood!
Anyways, you'll find Gakin in the basement of the Slaughtered Lamb in Stormwind.