What a great question. My first BDSM fanstasy was brought on by watching some kind of movie when I was younger and there were a bunch of boys being prepped to go into the military. They were lined up and all naked and there was a nurse inspecting them. I used to fantasize that was me. Having those boys and men bend over and I had a latex glove on and inspecting their genitals.
The first time I actually did any kind of act was when I was about 10, I had a boy who lived down the street and my mom found us in the back yard and I had tied him to the clothes line. I was teasing him with some toy or something he wanted. Little did we all know that 7 yrs later I would have my first boyfriend tied to his bed with handcuffs for hours while I teased him.
Well, the good part is that the fantasy above about the military men is still a big fantasy of mine & just a couple weeks ago, I was reminded again when I went and saw the movie, Notebook. Something wonderful about seeing a bunch of unsuspecting boys lined up naked with their hands in front of their cocks.