1. NGO, NPO, International Organization (the United Nation, World Bank, etc)
Working for these organization will give you the opportunities to live/work abroad on humanitarian missions (also choose to engage in the work based in the US) with a good benefit package. Generally, very competitive to get a job, but maybe not so for some NGO / NPO.
Regarding your study area, you have many options; medical, political science, economics (macro), math, statistics, public administration, lnternational laws, etc.. In most cases, you need an advanced degree (MA, PhD) to apply or be successful in the organization. Strong language ability is usually needed (e.g. French).
If you wish to learn more on the type of humanitarian works, this link may give you some idea;
2. Multinational corporations
Some works at private business, too may be close to what you have in mind. For example, you can introduce new products that improves the quality of life in other countries, build a new plant, that may improve the employment situation in the area, build a bridge, pipeline, powerplants, etc.. It may not be as humanitarian as NGO/NPO job, as you do business to generate money, not for welfare. But financially, often it is better than NGO/NPO jobs, and may be more fulfilling.
3. How to get the job:
There are many kind of jobs in humanitarian fields - field officer (who lives/works in the country to complete the given mission), analyst, program officer, fund manager, interpreter, administrative officer, etc, each requires different skills and academic background. To give you some idea, pls see;
(Pls don't get scared at their requirements or the complexity of the mission listed there - you are still very young with lots of lots of potentials!)
So, for now, you can just relax and concentrate on your current study, then in a few years, I think you will know which area of study you want to pursue - and if you still want to take a job at international humanitarian field, many doors will be open for you.