Do you have any previous martial arts experience? Odds are you will work on stances and basic boxing and some low kicks. Grappling will consist of basic positions of guards, half-guards, mounts, side controls, North/South. You'll probably get of straight conditioning as well. Take a bottle of water or half water/half gatorade. Wear a jock-strap and a cup and take a mouthpiece you've fitted. You can buy all that stuff at Wal-mart fairly cheap. Make sure you fit the mouthguard before class though. I'd take a comfortable t-shirt and shorts with a drawstring for grappling. I wouldn't drop the money on a nice rashguard until you know you'll be staying with training. Also, pace yourself as your emotions will wear you out on the first day.
Day two will probably be flying submissions and donkey punches.