The first forum I ever joined was a Naruto forum way back when I can't remember. I never made a single post there, actually. XD I just signed up and got lost and never came back.
The first forum I joined that I was actually active on was... ZeldaDungeon! Yeah, back in August last year I joined and actually posted quite a bit here. Granted most of those posts were pretty bad, admittedly, but I straightened up.