I was first exposed to competitive in January of 2012 via a Highlander pub competition between two "rival" groups (that used the same servers, actually), one of which I was a part of. Then, my group formed a Highlander team for that upcoming UGC season. I tried watching their games, but couldn't because of my stupid bedtime. A friend told me about the ESEA LAN, and I was hooked after watching. I instantly joined every competitive thing I could find. I started with Newbie Mixes.
Meanwhile, part of my group split off into its own group (with the HL team moving with them), and I followed them. I am still a part of that group today, but I rarely converse with them anymore. Last summer, some of them formed a 6s team, and others of us, including me after leaving the 6s team I already joined (I wanted to play with friends, and they needed a person) formed a "fun" 6s team. However, the HL and main 6s team both died, leaving this "fun" 6s team as the main team. We did decently. Two of us went off to play on another team for 6s, while I continued to play for them on their rebooted Highlander team. While I couldn't play on the 6s team as a starter due to the school year starting, I could play on the Highlander team. The next season, I tried getting the team back together but I was unsuccessful.
Last ESEA season, I joined the infamous Point n Click (now Donde) as a backup. I tried playing this season, but I didn't find a team in time so I've given up on playing until college, or when I can go to bed at 10:30/11 or later.
I don't know why I typed all this, just wanted to make a long post I guess.
TLDR: My TF2 experience has been pubs -> HL -> 6s. According to what Marxist said to me on Steam, this is how it happens to most people. Here's a pastebin of Marxist's and my Steam chat: http://pastebin.com/Z406hFbW