My first time firing a gun was a 20 gauge shotgun. It knocked me over (I think I was eight.) My second timewas when I was in the 6th grade at a school weekend camp. I put about 10 rounds through a bolt-action .22 of some sort. (Keep in mind up here many young boys start hunting by the 6th grade, so this wasn't unheard of, even in the early 90s when it was going on.) Then, I did a little bit with pellet guns in JROTC in high school. My first time firing a centerfire rifle was when I joined the Nat'l Guard, before I went to basic. It was February 1999, and it was annual qualification. They handed me an M16A1 and some ammo, went over the basics, and let me bang away on the little 25 meter indoor range we use. I scored expert, I believe. Maybe it was sharpshooter. In any case, I did really good, given my level of experience.
Aside from that, I taught myself. Learned the four rules after I got home from basic; never heard of "rule three" in the Army...