It has always bothered me that I'm not certain of the first game I ever attended. It might be (only because I know for certain that I was at this game) May 9, 1970 when the Angels beat the Yankees 11-3 on Bat Night.
https://www.baseball-reference.com/boxes/CAL/CAL197005090.shtml
I remember everyone in the crowd holding up their bats and it looked like a forest of bats. I don't remember much about the game, other than Andy Messersmith was pitching. He even hit a homerun that game.
I really believe I went to a game earlier than that, but I've never been able to figure out which one. That game in 1970 sticks out in my mind because it was bat night.