For me it wasn't a grand event. I believe I was in the 6th grade at the time so I was no older than 13. All I remember is talking with an online friend over Skype who explained that they were transgender and told me about what that was like. As I was listening to them I realized that I was too and accepted it instantly. I've been identifying as male ever since and was always excited at the prospect of becoming a transman. I still haven't begun transitioning but I look forward to it every day.
Finding stories online through Tumblr, Instagram, or DeviantART helped me find community and comfort. I've also had a transman friend in my life who already began transitioning before I met them. When I realized we were both trans, just in different stages, it gave me a lot of confidence.