You talk about what you *want* to do, but for placements of more than six months, organizations are going to require that you have a lot of experience: a professional skill that would be of value in the developing world and wouldn't take a job away from a local person, the ability to work (read, write and speak) in a language other than English, skills and experience that can lead to local people generating income, better feeding their families, improving children's health, etc., or that can lead to the transformation of key institutions, such as government, universities/schools or NGOs, and so forth. T
Short-term placement agencies charge volunteers, or require these volunteers to pay their own way (flights, in-country transportation, health insurance, accommodation, food, security, translators, training, staff to supervise and support them in their service, liaisons with the police and local officials, etc.).
Here is a web site that can help you learn more about the skills and experience desired by long-term placement organizations, and how you can gain that experience locally. It also lists the various organizations that don't charge for volunteer placement (but require a great deal of experience -- the average age of a UN Volunteer is 38).
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:
The cheapest overseas volunteer-sending organization in the USA that will send unskilled volunteers for short-term assignments (six months or less) that I've found is SCI - Service Civil International ((http://www.sci-ivs.org/).
I know *nothing* about them other than what's on their web site, so please don't consider the listing of this organization in my answer as an endorsement.