Need:
1. Gun
2. 5 mags. 4 is the bare minimum but the more the merrier, so you don't have to do tactical reloads in a long stage. I think IPSC has a 32 round cap, but steel matches take more rounds depending on how often you miss.
3. WATER
4. Holster. You will want a good belt in the future if you don't already have one.
5. Magpouches. Your pockets will do in a pinch but it sucks.
6. 300 rounds of ammo. Generally need 150. My rule is take twice as many as you will need.
7. WATER
8. Eyes and ears. Electronic ones are nice to have because you can hear the RO and other guys around you.
9.$30. Most matches run $25 but sometimes the qualifier will kick it up to $28.
10. WATER
Nice to have:
1. Cleaning equipment, but just bring a bottle of breakfree and your HK will be OK. I never bring any cleaning/maint stuff with me unless I am shooting 1911
2. Hat keeps brass out of your shirt and sun out of your eyes
3. MORE WATER
4. Back up gun. You never know.
5. Tagalog
I bring my gun, holster, 5 mags, 4 magpouches, a first aid kit and lots of water, my ears, and $30. That's it.