Wow what a good question! I think it was Warrior Forum where I used to give people advice on stuff. Stuff that I don't and won't talk about on here. I need a break from that, as this is how I make my living.
First forum I joined was BMF. I wasnt really that active on the site though. I posted every now and then but mostly I used the site to find other forums to join and other ways to make money online.
The first forum I joined was Rediff Q&A. There was no payment but enjoyed beihng there. It was at that Site that I made friends with someone form the UK and she even paid me a visit here in India.
That would be between Bizdustry and Trendri. I have forgotten which one came first. But I understand that Bizdustry has undergone some rebranding or merging.
It seems like it was this Bible discussion forum on Vbulletin software. It was around 2002. I was in my mid-twenties. It was a memorable experience because the place got hijacked by these extremists. They were very hostile to opposing views. I really didn't like their views, and I got emotional debating them, so it caused them to come out the winner despite how vile they were.
Anyway, I never messed with forums much after that until 2009 or something and I got on DigitalPoint.com. It was very active back then. It was a very powerful forum then, due to the fact social media hadn't taken over. Anyway, I got introduced into web development there and started to develop some successful employment blogs.