mpsgunner can probably clue you in on one or more of the bigger matches in the STL area.
Just remember one important thing about IDPA. It's not to be confused with training. It's a game. Some folks from IDPA HQ finally had to put an article out in USCCA magazine to cover just such an issue because of folks that used things like IPSC, USPSA, and IDPA as training when it is not and it creates inherent training failures when it is relied upon for that purpose. You can train yourself around it when you recognize the differences, which can make it quite fun.
Some of the scenarios can get absolutely crazy on the "what if" scale. So much so that a shooter can find themselves shaking their head at some scenarios. I've noticed that some shooters get their heads in the game so well that the scenario is secondary to looking for that target and hitting the Zero zones. They're truly dialed in.
Here's the rulebook, read through it and take note of some key areas with info you'll want/need to know. Commands and equipment are a couple of important areas. Some of the equipment stuff can really get confusing when it comes to what will pass and what won't or what class your firearm may fall into. Safety is the MOST important area, like understanding the 180 rule.
http://www.idpa.com/Documents/IDPARuleBook2005.pdf
When you get to your first match, expect to be amped up enough that you'll make mistakes. Everyone does and with certain scenarios experienced shooters can even get over-amped and make mistakes. Compete against yourself to start, then transition into becoming competitive with others in your division. If you can find a local to the event area you choose that is willing to tow you around and let you get a feel for IDPA as you learn on the fly, that seems to be one of the best ways to get folks into the game.
Welcome to what can be some of the most stressful, adrenaline pumping, fun you'll have!