I recommend 9mm for starting out in IDPA
1) Affordable platforms are available (Glock 17/19 would keep it simple, a BHP Clone like an Arcus is OK, but a good holster isn't easy to find...I've tried). Sig' makes some great 9's, but you pay for them. Also, most Sig's are DA/SA which must be in "hammer down" mode requiring a DA/SA trigger transition...can be learned, but takes some pracice. A CZ-75 is a good choice, but operational controls aren't a simple as the Glock, but again...can be learned with diligent practice. Holster availability is a big consideration because of the IDPA rules.
2) 9mm is affordable to practice with. Less expensive to run through the course twice if you are inclined to.
3) 9mm is pleasant to shoot, and easier to learn some of the more "advanced" things you need to do in IDPA that you don't get out of bench shooting in a lane (e.g. shooting from behind cover, on the move, non-dominant hand only shooting, even strong handed only shooting, etc.)
All that said, I've been shooting my Glock 22 in 40S&W because platform simplicity was more important to me than caliber at the outset. I expect to switch to ESP (will be shooting my 9MM CZ-75B now that I have a compliant holster) after the big regional event up my way in August.
Good luck!
CZ52'