My brother was always on the Bay12 Forums and I just happened to look over his shoulder one and saw a whole bunch of the people had ponies as their avatars. I was kind of curious of why they liked ponies. After that, I found out what a brony was, but hadn't really put much thought into it.
One day I was with a friend of mine and decided to see what all the fuss was about. I watched the first episode, but wasn't super impressed. But since I did want to find out what happened (it was a two-parter after all) I watched the next episode. I didn't think it was that bad anymore, so I kept watching more.
I bought a shirt with Rarity on it when I was at the mall, but that was before I was really a fan. I wore it to school one day and a kid at play practice asked me if I was a fan of MLP. I admitted it and he said he was too. (It turned out quite a few people in the play were Bronies).
I told the kid that I wanted to watch the whole thing but I had very slow internet and really couldn't. He took my flashdrive and began to give me as many episodes as he could fit on the flashdrive so I could watch them without waiting for the internet.
Eventually, I was hooked on the series, and I'm so happy that I decided to watch that first episode.