Well, this might be a bit long, but here I go:
I think the first time I ever came across anything Touhou was this Roxas fight I saw on youtube and the background music that was playing was Extend Ash ~ Hourai Victim. I thought that the song sounded pretty cool, so I made a mental note to look up the game it came from, but it slipped my mind, and I ended up forgetting to look it up.
Some time passed by, and I came across that automatic mario video that plays Kumikyoku Nico Nico Douga. I thought it was pretty cool, and so I looked up some of the songs that were in the video. After watching some of the series the video introduced me to, I gave the video another watch to see what else I could get into. I had thought the Marisa Stole the Precious Thing video was a little weird, but it was pretty catchy, and I liked it for some reason. Again, I made a mental note to look up Touhou, but it ended up slipping my mind again. Some more time passed by, and one of the people I subscribed to started uploading videos of him playing MegaMari. I noticed that it had the same girls from the Marisa Stole the Precious Thing video. I decided to download the game, since it looked pretty fun.
That's when one of the members here saw that I put MegaMari in my "Currently Playing". From there, he introdeced me to the fighting crossover games, and then the main games. I don't like getting into new series once I get into one that really like, so I've mainly been into Touhou ever since then, reading different fanworks, watching videos, listening to different music groups, pretty much everything. Although, I still haven't managed to pick an absolute favorite character =/