A passport is your official identification when traveling abroad and should be carried with you at all times. Make sure your passport is up-to-date and has at least six months validity left from the date of your planned return.
Make sure you have confirmed tickets for all legs of your journey, including return flights. Airlines will not let you board without a confirmed ticket.
Some countries require visas in order to enter, so make sure you have the appropriate visa before departing. Check the country's embassy website for more information.
Make sure you have enough local currency to cover your expenses while abroad. You can usually exchange money at the airport or in the city center, but it's best to have some cash on hand just in case.