While I'd played a little guitar, I started playing bass with others on stage the day I first played bass- Here's the story-
There was a local band (1976) playing country, some older rock 'n' roll, CCR stuff. Merle Haggard, Waylon/Willie, Hank Snow, etc. They didn't have a regular bassists and two friends of mine were doing most of the gigs with them. I cleaning up the wiring on one of the guys' spare bass (a refinished Mustang). and one afternoon he called me to inquire about it. When I asked if he needed it that night, he said "No, you do." I told him I'd only ever played with other people three or four times (on guitar) for faltering jams, never worked up songs, and didn't know most of the stuff they played. He prodded me, cajoled me, encouraged me, and told me it paid $50. I was making $200/week working for my dad's HVAC business so $50 was enough to push me into it. I showed up (I knew the guitarist at least) and was scared to death. I knew enough from guitar to play a basic Chuck Berry sort of boogie and to be able to find the root, third and fifth of a chord most of the time. I got through the night, and figured I'd never hear from them again. But, the singer asked if I'd be interested the next gig- four nights in a row at a local bowling alley, paying $50/night. Two hundred dollars sounded great so I really learned how to play bass on stage- and 'shedding with columns in Guitar Player Magazine by Carol Kaye and Chuck Rainey. I was coming from Cream, Allman Bros (the original band with Duane and Berry), Airplane/Hot Tuna, etc. When I was accused of being a "busy" bass player I didn't understand because what I was doing wasn't anything like Jack Bruce or Cassady...
That band had a lot of turnover, I worked with four different drummers in six months. The guitar player quit to form his own band, and after a while asked me to be in it. He found a great drummer and I was hooked at that point.