Make sure to to use proper manners when you are at dinner. People at the restaurant that aren't going to prom are going to look at you a lot and possibly watch you, so you want to present yourself in the best way possible. Eat over the table, and be extra careful to not get any food on your clothes.
It's a good idea to take with you a toothbrush, toothpaste, dental floss, and breath mints. You wouldn't want to show up to prom with food in your teeth.
It's usually expected that the guy will pay for the meal, but it's always a good idea (girls) to take your own money with you, just incase. Also, when estimating the cost of the meal, make sure to estimate the cost of the tax on the food and the tip. Good tips now run around 20%, so make sure to think about that ahead of time. Also, if you go with a group (usually seven people or more), gratuity will be included in the bill. This means it's NOT optional! Make sure everyone pays for their part because it's no fun and not fair for the last one at the table to pick up everyone else's tab.