As soon as you enter, you'll run into the owner of Join Avenue, and he'll talk about a whole bunch of stuff regarding the place and then ask if you'll help him fulfill his dream of expanding it. Go ahead and say yes.
At this point, you'll then have to enter you're favorite phrase, although keep in mind that it will be visible to the whole world. Afterwards, you'll have to enter some words to say when you are impressed.
After you're done with that, it'll ask for confirmation. Go ahead and say, yeah, sure, then the owner guy and his associates will leave, leaving behind the two Ace Trainers. One will ask what your title is, so go ahead and enter something that everyone should call you, then confirm.
You'll also have your own office building set up just to the left of the Ace Trainers.
There's plenty more to do in Join Avenue. I'll come back to this later and cover it, but for now, know that you can check back often and that interacting with other players with your C-Gear on is a good way to build your avenue.
Anyway, that's probably all you can do here for now, and I'll cover this more later on probably, so just go on head north into Nimbasa City.