Can't say, it's been years. The beginning is so hazy. I only remember the moment where I decided to watch it, but I don't remember my initial reactions. I think it started when I actually decided to watch the show.
You see, at first I saw all these groups on this one site about MLP, and while looking through their chat they were all talking about "cupcakes". I decided to google it and I read the story, then somehow I saw bits of "party of one" so I decided to watch the whole thing. I think that was the point I became a 'brony'.
That's the shorthand of what happened just for the sake of answering the question.
2011, good times. Good times.