Find Kranal Fiss in the Barrens.
It's been some time since you were tested last, [player]. You've been patient thus far in your studies, but it's time for you to learn about greater powers... it's time you learn real power. The element of fire is yours if you're prepared to pay for it.
Seek out Kranal Fiss in the Barrens. He lives in a small dwelling north of the Crossroads, but be mindful, if you've reached the blasted night elf lands, you've gone way too far.