Three things:
Hearing the bass in Seasons of The Sun by Terry Jacks, I just loved the sound, no idea what it was but from then on I listened out for it in other songs.
Seeing the “live” bit in the Bohemian Rhapsody video by Queen, I knew at that point I wanted to play music when I got older (and I could also hear John Deacons bass, that sound again).
Hearing Pretty Vacant by The Sex Pistols, I now knew what type of music I wanted to play.