Just know that your there to have fun and don't worry so much. You need to relax on vacation so just have fun.
Don't forget sunscreen (did you know that your more likely to get a sunburn on an overcast day than on a sunny day???) and insect repellent. Also you may need a First Aid Kit.
If your...
Bring clothes for warm or cold climate (depending on where it is your going)
Food (hot dogs and hamburgers are best for dinner; and eggs, sausage, and toast is best for breakfast; sandwich stuff for lunch)
First aid kit
Tent
Hammer
Sleeping bag and/or blankets
Sleeping foam or air mattress...