The first actual game I played was a home brew game. I talked about that a big a few posts ago. To summarize it was something conjured up from the minds of 10 to 14 year old adolescents. Replete with hilariously improbable combos, zany ecology, laughably politics, etc. in short it was splendid fun! We played for about ten months I think in our home brew world. Then a few PCs died, and we had some transition in the group. We were kids, totally different kind of game when your a group of 10-14 year old kids vs. adults.
Anyway the first published campaign setting I played in was actually the Mystara stuff. I fondly recall a generic start
I believe we did some of the Karameikos stuff, we went to Glantri, did more home brew stuff in Mystara and I think our DM used some other modules too and subtly disguised them. He never really told us when he was running a module or not. I was ten and eleven years old and didn't really pay much attention to that sort of thing. I was more concerned about what my character was going to do next and whatnot. Its honestly been years and years since I even thought about this stuff ... lol. Those were the days though, the golden age of RPG adventure for me anyway.