For me it was a friend from little league baseball. Here's my long story told as short as possible. In 10th grade wood shop we were allowed to pick our big semester project. 15 year old me told the teacher I wanted to build an electric guitar (I'd been learning on acoustic for a couple years). He realized it would be overly ambitious so suggested I build a bass instead since it only had four strings.

Not knowing anymore then he did and really wanting to build something in the general shape of a guitar I agreed. Fast forward a week or two and I'm walking out of shop class with the rough cut body of the bass under my arm. Taking it home for the weekend to show my dad and my neighbor friend who helped in the shape design. My baseball buddy, Dale, sees me leaving shop and says "hey is that going to be a bass"? Yep, I reply. "You play bass?" he asks. Nope, I reply. He said that's too bad we need a bass player in our band. I asked him if I was able to get a bass could I be in the band? He said yes but you'll need an amp too and we'll show you everything you need to know.
That afternoon when my Dad got home from work I told him I could be in a band if I had a bass and an amp. He said aren't you making a bass in shop. Well yeah, but I can be in a band
now! So after dinner and convincing my mom it would keep me off the streets and off drugs,

my Dad and I went to Guitar Trader in San Diego and I got my first rig. A Peavey T-40 with a small Kustom combo amp. Thus a bass player was born. I joined Dale's band and I was gigging within a year (with a different band) and I never went back to guitar. 41 years later I'm still bassin' and gigging. And only marginally better.
I only wish I had expressed to my dad before he passed how much that $250 investment in my musical journey has meant to me. The opportunities and great times it has provided and the profound influence it has had over the course of my life.
I bet he knew.