Some towns are set to [open] which means you can join on your own by doing /t join [name of town] but some towns are not open therefore you need an invite from a town staff member in order to join. My suggestion is just to go around to different towns and see which one fits best for you. I can tell you the most active...Windermere (which is my town) is #1 in residents but Ethereal is the next most active town. Again, just do /t list, and just town tour a little and see what you like.
-gg