I've been a part of many internet forums in my life, but the first one I was ever a part of was the Super Skarmory forums. Which was a forum for Nintendo/Pokemon fans to gather who were fans of the then YouTuber, Super Skarmory. Those forums have been long since dead but I'm in contact with a good amount of people from that forum today. One of those people I've been trying to get them to join TBT!
The first forum I ever joined was the Ushi No Tane (Harvest Moon) forum back in 2008 or 2009. It sparked my interest in sprite edits of all things, which then got me into pixel art! (And graphic design with signatures-- there were some shops where you could request them and some people made them with gifs and stuff? It was very cool and I was determined to learn and be like them haha.) It's very nostalgic to think about!
Mine was a forum called friendcodes.com in 2008. It was a basic friend code exchanging forum that started during the Nintendo DS era. It changed it's name to nintendoplus.com but it shut down, unfortunatly. One if my old mutuals there actually introduced to THIS forum though, in 2014. It's sad to see a lot of forums in general die because I prefer them so much more than the annoying social media sites these days. It makes me greatful for TBT because it's still thriving to this day compared to other forums.