I was playing on stage before I even knew how to play guitar. The first band I joined didn't have a bass player so I tuned my guitar down really low and played single notes on it. Next band I played with needed a bass player for a show they were doing & I didn't even own a bass, so I rented one... didn't know any of the songs so I just watched the guitar player and faked it.
I guess the point is to start trying out for bands as soon as you can. Even if you don't get the gig you learn what's needed and you get over your shyness at playing in front of other people. And best of all, even if you do get the gig and you suck, you still get the groupies cause most of them are tone deaf.