For the travel virgin still in college (who knows they want to travel as a career), I will not direct you to a specific major. I majored in International Studies, which was good for the framework but I really don't want to discourage those majoring in subjects like teaching, engineering, chemistry, biology, premed, nursing, business, environmental science, or the arts. We need EVERYONE. However, if you have room, I highly recommend that you minor, because the theoretical framework is essential. If you cannot minor, at least take classes in world politics, U.S. politics, and basic economics classes. These are vital courses to understanding relevant political systems, the market structures in place, and the reasons why countries interact in the manner they do. Seriously, I know politics don't appeal to everyone and I know Econ can be dry and sometimes very difficult (trust me, I minored in Econ and it was HARD), but you will be so grateful you took these classes. Also, gain proficiency in at least one modern foreign language. Everyone says Arabic or Chinese but even Spanish or French will help you. Also, follow the news. Kind of helps if you know what's going on.
Not in college or don't have the time? Educate yourself. You can get basic economics books and books on politics from your local library. Follow the news. Follow ALL kinds of news. Invest in a Rosetta Stone or take summer language classes (even audit if it's cheaper, they don't care about the grade, they care if you SPEAK it). Even if you don't know where exactly you want to go, learning a language that's spoken by a wide range of people can help (and learning one language can help you learn more). Having a basic understanding of the world stage and having language skills are vitally important.
For the very basics, I'd stick with this. Regardless of your reasons for traveling, understanding why the world is the way it is and having proficiency in at least one language helps A LOT.
Stay tuned! There will be more to come!