It wasn't my first one (I went to a few earlier in the year) but probably my most memorable one.
Padres vs Phillies, September 3rd, 1989 (I was 8 years old)
Andy Benes' rookie year and he was pitching that day.
Andy Benes was up to bat and I remember yelling for him to get a hit (Because I knew pitchers weren't good hitters, so I was hoping for a "lucky single") and, instead, he hits a home run!
Padres won 9-5
I was also an inch away from catching a Tony Gwynn foul ball, but it went right by my glove.
Here's the box score
September 3, 1989 Philadelphia Phillies at San Diego Padres Box Score and Play by Play - Baseball-Reference.com