My first forum was VGMusic, and it was basically where I learned forum etiquette, and how forums even work.
I still frequent that forum, even though not much goes on there. When something does go on, though, it's usually pretty good!
From there, I joined Smashboards, when I got into Brawl hacking, and was quite active there, with lots of plans, and a few things to show for it. I quit after I realized that for the amount of time it took to make something for Brawl, it really wasn't much worth it.
Somewhere in there, I joined YoungComposers, and proceeded to search for my place there, but I got sick of the many grumpy moderators there who have nothing better to do than complain about newbs and Christians. Every day. I'm not joking.
Then, I joined here, after playing a bit of Tortured Planet. I didn't plan on getting very involved in the community, or ever designing anything for it, but I guess I ended up doing all that anyway! :p