For anybody who has never shot an IDPA match...
You need to dress such that you will be comfortable outside for up to 5 hours. Most matches are 3 to 4 hours.
You need to bring water and a snack.
You need to have cash for the match fee. Most places cannot do a credit card.
Assuming you shoot something other than a revolver, you need three magazines min for your gun. If 1911 or BUG, you need 4. I take 5 or 6 just in case of an issue.
You need some kind of magazine holder so you can have 2 mags stowed on your belt. You can use your pockets but you will be unhappy.
You need some kind of cover garment that covers your holster/gun and mags when your hands are extended. Vest, jacket, coat, long shirt, etc.
You need a holster that securely holds the gun and is concealable (see immediately above). No crossdraw, ankle, shoulder, drop leg, or appendix holsters. Pretty much has to be strong side either in the waistband or outside the waistband.
You need eye and ear protection. Regular glasses or sunglasses are acceptable.
You need to bring around 120 rounds for a normal match. You will likely only fire 90 but you might need to reshoot a stage or something. If you shoot a classifier you need another 100 rounds to be safe.
I bring other things but you would be well prepared with the stuff I listed above.
For USPSA or Steel the list is a little different but not by much. Mainly more ammo.