Find out what they've got and what they recommend. The answer to "what should I use" will also depend on how far you're going to be shooting. Do you know the answer to that?
ETA: You're young, so I'll also add to be sure to be a "good guest." Offer to clean the rifle and put everything back nice and neat. That's a good way to help ensure that you get invited to shoot again.
A gunfight is not the time to learn new skills.
If you ever have a real need for more than a couple of magazines, your problem is not a shortage of magazines. It's a shortage of people on your side of the argument. -- Art Eatman