Mine was "In Good King Arthur's Day" in front of a very kind and supportive audience who, like me up to that point, had enjoyed joining in the choruses.
I had to teach my first ever music lesson three days later and the club let me try out the song on them first.

Next week I was back with my second song....
and I was hooked.

Typical Fat B*****d Rock and Roll - I should have guessed *grin*