Interest in competitive TF2:
eXtv/SalTV/vanillaTV. First channels that showed up when I searched Competitive TF2 on Youtube at the time. That was about 4 years ago
Starting to play "competitive" TF2:
TF2center after watching streamers play it. I moved away from centers eventually and instead starting to somewhat lead a mix group. The mix group still plays on rare occasions
Being on a team:
ETF2L S28 is my first season. I started in Mid. I got lucky enough to encounter my current teammates in a faceit lobby (edit: Should also mention I played like 5 faceit lobbies. So I got very lucky). They liked me and the same night asked me to merc for a scrim. The next season I started playing with them.
No real reason why I was never on a team before. Never bothered to look for a team because I wanted to play with people I know and like. Luckily I get that chance now :)