I was apparently late in starting to masturbate. Not having an older brother or friends to teach me, I had to discover it on my own.
During one summer (I was in high school at the time and I may have been 15) I was riding my bike around the high school on a Sunday afternoon. I'd always had a fascination with locker rooms, the smells, the sites, etc. The school was locked up but I noticed there was an opening in a fence that closed off the parking bay. Directly behind the bay was the door to the men's locker room. I parked my bike and found that I could squeeze through the opening of the fence. I quickly walked to the exterior door of the locker room and to my surprise, found it to be unlocked.
Once inside I realized that since it was a Sunday, no one was around. I walked up into the gym and then back down into the locker room. It was very exciting being there with no one around. I was free to do anything that I wanted. Cautiously, I picked my locker and started to undress. It was incredible to be naked in the locker room and I quickly got a hard on. I walked around for a while, ran up the steps to the gym and then started to rock back and forth on the bench in front of the lockers. It was so hot to do all of this naked.
It felt real good and it didn't take long before I realized something was going on. I knew what was about to happen but never having had an orgasm before, I didn't know what to expect. I quickly ran to a urinal just as I erupted. It felt incredible and to this day I still think my first orgasm was my best.