If you already have a copy of PSO for the Gamecube and a GC or modded Wii, then sign up to https://schtserv.com/forums/app.php/welcome

It's free and they're almost always some open parties available for people to join (or create your own team and randoms will almost certainly join and help you level up).

I'm usually around on Tuesday evenings (UK timezone) but the guild meets other days too.

Do you have any characters already leveled up? Or would you be starting from scratch?

what version do you play? I believe that affects it. There are custom servers for pc and gamecube, but there's more work on dreamcast