For Iron Maiden, it was in English class and we were discussing about the ballad, rime of the ancient mariner. The teacher made a reference that this band made a song about it. In grade 9, I thought, "Hey, that's fuckin awesome."
I got home, went on Napster, and downloaded a whole shitload of Maiden tunes. The first song I heard was Hallowed Be Thy Name, and I have never heard something like this in my whole life. After buying The Number of the Beast I was instantly hooked.