Yup, it's hard. The most valued training for work in the developing world is undergrad training -- yes, a four year college degree -- and even a Master's Degree. You will find that most development jobs listed at places like ReliefWeb and DevelopmentEx want that level of expertise.
Communities in the developing world need people who are civil engineers and architects who know how to design and build using only local materials, people with business degrees who can help communities set up viable co-ops, education professionals who know how to set up viable education institutions (not just a one room school here or there), legal people who can help set up policies and laws, medical professionals who can help start large-scale prevention treatments or prenatal care, agriculture specialist who know how to introduce new crops that are appropriate and profitable in an area, etc. This level of expertise requires at least a college degree. And, ofcourse, one person can't do all of the aforementioned; you need to pick one area and concentrate on it (and, in addition, you need to be able to work in another language).
There are some things you can do now with regard to volunteering that can help you gain experience needed in the developing world, such as helping with elections in your own area, working with programs that help immigrants gain knowledge for and access to computers and the Internet, helping with programs that educate young people about HIV/AIDS, etc. Such volunteering will help make you a more attractive candidate for such work, in addition to your degree in a subject needed in the developing world. Here is a web page for more advice on gaining this experience:
Otherwise, as an unskilled person, you can pay for a short-term experience in another country, to get an idea of what it is you want to concentrate on. You need to be prepared to pay for your flights, in-country transportation, health insurance, accommodation, food, security, translators, training, staff to supervise and support you in your service, liaisons with the police and local officials, etc.
There is a listing of the more-than-30 member organizations of the International Volunteers Program Association (IVPA) that is a good place to find reputable volunteer-for-a-fee programs -- programs where you don't need to have much experience in order to participate, and the placements are just for a few weeks or months:
You can also try United For Sight. The goal of Unite For Sight and its partner eye clinics and communities is to create eye disease-free communities. "While helping the community, volunteers are in a position to witness and draw their own conclusions about the failures and inequities of global health systems. It broadens their view of what works, and what role they can have to insure a health system that works for everyone..." This program was featured on CNN International. Volunteers, both skilled and unskilled, are 18 years and older, and there is no upper age limit. It is obligatory for accepted volunteers to purchase insurance coverage through Unite for Sight's recommended provider, and volunteers are responsible for all travel arrangements, visa vaccine requirements, lodging, airfare, food, and any additional expenses. http://www.uniteforsight.org/intl_volunteer/