The other guys, especially KZ45, have given you some great advice. I’ll try to add just a few things from my experiences.
Kneepads, as recommended, are a must-have item. Elbow pads are optional, depending on the drills you’ll be doing.
A good tactical sling that allows you to position the rifle downward (pointed at the ground) is another must-have item. A sling that allows you to transition to your weak shoulder might be of added benefit.
Only use good quality magazines and good quality ammo. The classes I’ve taken all used 30-round mags. Your class might be different. Get a LULA for quick re-loading or use stripper clips.
Make sure your rifle is zeroed before the class. Make sure it is clean and fully functional before the class. Bring a cleaning kit and maintain that rifle during breaks. Most guys will stuff their face with a sandwich during the breaks. Take the time to check your kit first. Having spare parts is not a bad idea. In my case I brought a spare rifle. [:D]
Bring water. Have some water on your rig so you can hydrate in the field. If you have a camelback, use it. If not, canteens are fine and they are cheap. Power bars, as suggested, are a great idea.
I echo the comments about comfortable clothing. I usually wear BDUs (basic black and khaki [;)]) and leave the cargo pockets open so I can drop empty mags in them. You will need a place to drop your empty mags. Comfortable boots and some spare socks are a great idea. Keep your feet dry and comfortable and you won’t become fatigued as quickly.
Put together your web gear now and make sure it is balanced. If you don’t already have any web gear, don’t spend a huge chunk of change of a high-dollar vest just yet. A basic H-harness and padded web belt will do just fine. Get a couple of mags pouches and canteens. Make sure everything is balanced. (Yes, I said it again.) Best thing to do is put your gear together and move around in it to see if it’s balanced okay. I did 2 and 3 mile aerobics videos in all my gear (even with my unloaded rifle) for two months before the first class I took. My wife got a big laugh out of it, but it really helped me decide what worked and what didn’t.
To be honest, I’ve seen guys just stick mags in the back pocket of their jeans. It depends on the drills you’ll be doing.
A pen and notebook (the small pocket-sized kind) aren’t a bad idea to take some notes.
Great idea to avoid ear plus; use muffs instead. But, if you have some earplugs and you do NOT have a gapper on your rifle, bring one earplug with you. It WILL end up in that gap between the pistol grip and the trigger guard.
Just relax and have a good time. Take your time at first and get your hits. Speed will come later.
Enjoy!