I'm unsure of what my first gaming experience was, as there are many games I remember playing when I was very young but don't remember which was first.
If I had to make a guess, I think Doom was the first game I ever played (first person shooter). My dad had it on our very old computer, and I think he showed me how to play it one day. I believe he showed me the cheats iddqd (invincibility) and idkfa (full ammo and keys) so that I didn't die and the game wasn't too difficult for me.