1. Good comfortable holster that you'd use for everyday carry
2. spare mag holder
3. eyes and ears
5. practice time
You'll love it. Check the IDPA website to make sure your holder is OK, and to figure out which class you'll be in..depending on pistol chosen.
Go visit one of the IDPA matches in your area, but take your gear with you. It's no fun to watch...get in the game. Some places will let you shoot a match or two without joining IDPA, but I think it's up to the individual clubs.