United States citizens must have a valid passport to travel internationally on any airline. Some countries require travel visas for tourists, though visa regulations vary by country. You'll need your passport to reenter the United States at the end of your trip.
Many countries regulate liquids on flights. While specifics vary by airline and country, you should limit personal toiletries and liquid medications in your carry-on luggage to no more than 3 oz. and seal them in a clear bag with a zip-top. Delta Airlines suggests having these liquids out while going through security, which could speed up the process.
All airlines require proof of parental permission before allowing an unaccompanied minor to fly. Many countries and airlines require a legal guardian or specified party to pick up the child upon arrival. All of the visa, passport and identification laws also apply to minors, meaning they'll have to show as much identification as an adult to board flights and visit some countries.
Because each airline has different policies, the requirements for international flights can vary. Contact the airline you'll be using for specific requirements and regulations.
