It's nice that you have enthusiasm and want to help others. What people in need of humanitarian workers want are people with more than good hearts, however: they want expertise. They need people who can help train them for work, who can train them to be their own nurses, doctors, teachers, dentists, accountants, communicators, etc. Humanitarian work has shifted greatly in the last few decades from "We're here to do what you need" to "We're here to help YOU do what you need." In many places after a natural disaster, for instance, the first thing done isn't to bring in lots of workers to rebuild but, rather, to fundraise so that local people can be hired to rebuild. So if you want to work in humanitarian scenarios, think about a skill or expertise you want to master. Also, get involved locally in the kind of work you want to do internationally.
I have a web site that offers very specific advice on how to pursue a career as a humanitarian worker, such as with the UN: http://www.coyotecommunications.com/stuff/workabroad.shtml