Qualifications
Once upon a time, international sales positions didn't necessarily require you to have an undergraduate degree. So long as you could sell, sell, and then sell some more, employers were willing to overlook the fact that you hadn't been to college.
While there are companies today that would be perfectly willing to hire you without a college degree, so long as you have a demonstrable talent for sales, you're likely to find that corporate HR departments automatically discard résumés from candidates who don't have degrees when an international sales position specifically calls for an undergraduate education.
There may be hundreds of people applying for one international sales job. That degree requirement gives HR minions a way to easily whittle down the talent pool. Unless you're some kind of sales prodigy straight out of high school, you'll need to earn, at the very least, a bachelor's degree in business if you want to pursue this career (source).
There are a variety of majors to choose from when it comes to a business degree, including accounting, finance, management, and marketing. While an employer isn't likely to be too particular about your business major, if you're certain that international sales is the career for you, you might want to enroll in a university that offers a major in international business.
While a major in international business will load you up on the foundational courses common across all business degrees, this particular major will also require you to complete upper-division coursework, particularly in a foreign language.
Proficiency in a foreign language and knowledge of other countries and cultures will make you extremely competitive when it comes time for you to land your first job in international sales. If you choose to attend a university that doesn't offer a major in international business, you should explore taking foreign language and regional studies coursework on your own.
You may also find, once you enter the workforce, that some international sales jobs require a master's degree, usually an MBA. Keep in mind that if you choose to pursue a graduate degree in business, you'll benefit enormously from having first acquired work experience. Furthermore, many MBA programs are heinously expensive, so you might want to find a creative way to fund your graduate degree if you choose to pursue one (like going to work for a company that will pay for it).
One last note: While studying other countries and cultures in a classroom will give you some useful theoretical background for a career in international sales, employers will be more impressed if you take what you learn as an undergraduate and gain some practical experience abroad, either through a study abroad program or through an internship.