This seems to come up from time to time so here we go. I've shot a lot of these things and help run a monthly match so I think I might know what I'm talking about.
1. Read the rules! For the most part there is no standard set, so everybody's different. This can save you a lot of grief. Also, how physical the match is plays a part in equipment selection as well.
2. Holster, something that will securely hold the weapon. If you shoot IDPA, then the same gear will probably work. I prefer a Safariland b/c I shoot matches with multi-gun stages where we reholster a hot pistol. The retention makes me warm and fuzzy. Watching someone reholster a hot gun into a nylon leg rig doesn't do much for me.
3. Mag pouches. Pistol pouches are mandatory. You can usually get by with a cinch-mag for most rifle stages. If nothing else put a spare in your back pocket.
4. Slings. Depends on the match but better to have it than not. You can always take it off.
5. Knee pads and such. Again, depends on the match but they can be nice to have. Long pants, wear 'em. Temperature be dammned.
As far as weapons go I will make no recomendations(glock, ar, benelli) except that they need to work. And YOU need to know how to make them run. I've seen a lot of strange manipulations.
I see a lot of people seemingly afraid of showing up with the wrong gear. News flash, shooters win matches, not guns and gear! Go and check out a match and ask questions. If there is one thing we all love to do its extoll the virtues of our latest purchase. Then you can make an informed decision instead of going off what someone who has never been to a match says. You are not going to win the first match you go to no matter what you buy. When you do buy something(and you most certainly will) buy quality gear. If noone else seems to be using it, hmmm maybe it not that good. There are points to be bought in this sport but the skills have to be there.
Also, unless it's a part of your duty gear, if it doesn't help you shoot faster and more efficently then it has no place on your person. I see a lot of rambo knives and whatnot. Whatever makes you happy I guess.
Hopefully this will be of some value to somebody. This is a great sport and a lot of fun. Good luck and good shooting, Gary Byerly.