TOD
'Tis with regret that I say that there's nothing theater-wise that had its effect on me in a way that I ever thought I remembered every detail of the experience, or anything close to that. But since, to the very best of my knowledge, a local performance of SOUTH PACIFIC was the first musical I saw on stage (probably first play, period), and I was blown away by the magic of the piece (the performance might have been execrable for all I know), I'll credit that night at the War Memorial Auditorium in Fort Lauderdale with being printed indelibly in my memory, and planting the seed.
It was a few years later that I volunteered my services as rehearsal and pit pianist for the Junior Theatre's production of LI'L ABNER. While that, too, was most assuredly nothing to write home about (I already was home, anyway), that experience truly lit the fire.
So, those two are significant for being "firsts", but that's about as far as it goes. And I'm not sure I even answered the question.