I use my SR9 for CCW and IDPA. I used to use my carry rig holster, a Crossbreed Supertuck, for IDPA, but I recently got a Comp-Tac kydex OWB that works much better for competition.
The M&P has gotten very popular at the local monthly match I attend, with a lot of them being former Glock owners. Lots of people shooting 1911's, a few wheelgun shooters, and a smattering of other makes and models.
There's some beginner's info on our local group's Web site:
http://www.pmrpcidpa.com
The type of gun you choose will determine what division you shoot it. For example, I shoot my SR9 in Stock Service Pistol (SSP). Any gun that qualifies for SSP will also qualify for Enhanced Service Pistol division. If you continue with the sport, you'll also shoot a Classifier at some point to get your classification (Novice, Marksman, Sharpshooter, Expert, Master). You compete against others in your division/class.
You'll need a gun that can conceivably be used for CCW (no 1911 race gun with compensator, red dot sight, and toilet-sized mag well), a holster that can be used for CCW (IWB or OWB), and mag pouch(es) to carry two additional mags.
It's easy to get into novella length telling you about it, so just check out the link above, the main IDPA site, and if you have any questions, ask away.
-JT