McAllen-Edinburg-Mission, Texas AKA the poorest city in America. Home to a 12% unemployment rate, plus more than one third of the city is below the poverty line. This would be my first stop. There's alot of poverty, which means people need serious help. I'd provide food, clothing, and any other neccessary products. Because there are so many unemployed residents here, people who are willing to assist others are in high demand. I would be truly needed and useful because of my service-oriented attitude. There aren't many other humanitarian organizations whose focus is on the United States, so I would be making a huge difference.
This city may be the poorest in America, but there are much poorer countries in general. there wouldn't be enough to do there, so I'd leave McAllen-Edinburg-Mission for Haiti.