The most important tip ? Avoid unnecessary stress by applying for a passport well before your expected departure. Don’t book a flight or make any arrangements that can’t be changed until you have your passport in your hand. Some international flights require that you enter your passport number when booking or during online check-in, so it's best to just have it first before you start booking anything.
As you book your trip, make sure you check to see whether or not you will need a visa to travel. A visa is a document issued by the country you are visiting, which grants you permission to travel there for a certain number of days. For most of Europe, the U.K., and many countries in Asia and Latin America, most passport holders can travel without a visa or receive one on arrival. If you do need a visa though and you show up for your flight without it, you won’t be allowed to board the plane.
Make sure to double check your allowance to know what you can carry with you during the trip.
It's wise to give a copy of your itinerary to a friend or family member. Also don't forget to notify your bank of your destinations. Also check about international fees, and whether they have partner banks in the destination that will help you save money on costly "foreign transaction" ATM fees.