I figured here would be a good place to start, as I'm going to do it in real life soon and this is the least potentially harmful place to do it. I'm a pre-transition MtF transsexual, and while sexual preference doesn't come up often on this forum, I figured I should clarify I'm attracted to girls.
In real life, I've told my parents (but not my only sibling, my older sister), and a select few friends. Everyone's been supporting so far, and I hope it stays that way, but there have been varying degrees of support-ness. I'm currently 16, and have acknowledged I felt this way for about six years, give or take a year. Looking back, I now know that I've felt it for much longer, but never really formed the conscious thought "I'm a transsexual.", or even really knew why I felt the things that I did. Also, it was more of a gradual realization. There was no "epiphany moment", per se. All I remember is one period of time when I didn't realize it, and a period of time where I did, but where one stops and the other ends is sort of a blur.