Not counting mostly vanilla type of tying to the bed by my wife years ago, the first submissive act for me was just very recently.

I was ordered to strip naked in front of a beautiful lady I had never before laid eyes on. Granted she was a paid Mistress but it was still me giving up all my rights to privacy. It was thrilling and proceeded from there to other activities. I get excited just remembering the lose of control and facing a beautiful stranger with noting on but a smile.