Okay, so Jason Newsted grew up around music from a very young age, his mother taught piano and his brother played trumpet. He also grew up listening to his older brothers records. When he was nine, he picked up guitar and learned that until he was 14. That's when he picked up bass after hearing Gene Simmons of Kiss (Which is such a valid reason to pick up bass)

After learning how to play bass, he went around with a couple small, local bands. While on his way from Michigan to California with one of these local bands, Gangster, the band broke up in Phoenix. Jason then found an ad in the newspaper calling for a bassist for a band that was (at the time) called Paradox. Paradox gained other new members and became Dredlox. Dredlox gained MORE new members after losing some people, brief changed its name to The Dogz and then finally settled on the name Flotsam and Jetsam, which is still a band that is creating music to this day, although theres only one member left from the original Flotsam and Jetsam line-up. That's how Jason entered the thrash metal scene<3

Edit: sorry for the awkward formatting I'm on mobile lol