I was a casual genre viewer as a kid, mostly just the Universal classics and Godzilla flicks. But I spent the weekend at my best friend's house in 9th grade, and he was a massive horror fan, and treated me to a horror movie marathon. From that weekend on, I was hooked. I don't remember every movie we watched that weekend but some were:
The Toxic Avenger
Friday the 13th Part 2
Evil Dead II
And there were several more, though they escape me at the moment. But I'd say it was those which cemented me as a horror nerd.