Ask your teachers, group leaders or advisers for a list of things you can and cannot bring on the trip. There will be some restrictions, especially if you are flying on an airplane.
Buy or get a suitcase that will be large enough for the number of days you will be gone. There may be a size and weight restriction on your suitcases. Check which type of luggage you can take on the overnight trip.
Check the weather forecast for the location where you are going a few days before the trip. Based on the weather forecast, decide which type of clothes you want to take. Regardless of the weather, bring a jacket.
Pack your clothes in the suitcase in the order you will be wearing them. This is nice to do, especially if you are going to leave your clothes in the suitcase for the duration of the trip. Put all of your underclothes, socks and pajamas in one area of the suitcase (such as up in the pocket of the lid). Pack all clothes such as tops, bottoms, underclothes, socks, shoes and hair pieces.
Pack all of your toiletries such as shampoo, toothpaste, hairspray, hairbrush, toothbrush and deodorant in large plastic bags and put them in the suitcase. This will help prevent them from leaking onto clothes. You can pack them into a smaller suitcase if you have more than one suitcase. Regardless, you should still pack them in plastic bags.
Include items to keep you busy during down times, such as a personal music player or a book.