I found it easiest to start with my local IDPA club. Mine had a lot of good info on how to get started, including a very good (mandatory) clinic. After shooting my first actual match today, I can say that the clinic was invaluable. You can read a few tips and insights here: http://www.glockforum.com/forum/f31/idpa-clinic-beginner-insights-3938/
The main IDPA site should be able to direct you to your local club contact.
The main IDPA site should be able to direct you to your local club contact.