I've only been to a few competitions, and I have yet to compete in IDPA, though I have observed a couple of their matches. One thing that all of the competitions I have been to have had in common is that everyone is friendly and extremely helpful.
At the meeting before the match, they will (I can almost guarantee it) ask if there are new shooters. They'll probably talk to that group a little longer and make sure that some experienced shooter in your squad knows you're there and to look out for you.
If you have found ammo that hits where your sights are pointed, stick with that. FMJ is a good choice, and some matches require it if they include steel plates as part of a course of fire.
Be prepared to meet a bunch of people and to have a lot of fun.
The only words of caution I have are:
1. Read over the rules before you go, especially about what is and isn't allowed for your gun. For example, you mentioned a G23. If it's a C model (compensated) you can't shoot it. If it's a stock, non-compensated model, it should be fine, but double-check.
2. Go slow. If you get in a hurry, it's easier to do something you can get disqualified for (like drop your gun or accidentally point it a little past 180º).
"An armed society is a polite society." — Robert A. Heinlein
"After a shooting spree, they always want to take the guns away from the people who didn't do it. I sure as hell wouldn't want to live in a society where the only people allowed guns are the police and the military."
— William S. Burroughs