I am cheating a little bit as I did not really buy my first RPG. However I didn’t buy anything after my second RPG for a very long time.
After I get the basic D&D set, some of my friends at school told me that I really wanted to be cool I had to play advanced dungeons and dragons. So, I bought my own copies of those books and other materials related to them. So my second RPG was AD&D, now referred to as first edition.
Funny thing is, later on in life I discovered that I like basic so much better.