I had heard the occasional (hard) rock tune by AC/DC, Deep Purple, Van Halen etc., but it were two different riffs that got me hooked.
The Owner Of A Lonely Heart opening riff, before the band comes in and that Money For Nothing riff, after the keyboard/drum build-up. I was 14/15 at the time and while both songs/riffs weren't really crushingly heavy, those riffs drew me in. They sounded hot, had a hint of rebellion and just flat out blew me away.
That's when I realized, that the electric guitar was the coolest instrument on earth and I went actively searching for songs with cool riffs.
First came the hard rock bands, Van Halen, Aerosmith, Def Leppard, Whitesnake and the older ones like Led Zep, Deep Purple and so on, and that led to metal with Anthrax, Slayer, Iron Maiden, Metallica.
And to this day I get goose bumps when that opening riff of Money For Nothing is played.