If it religious missionary work you're after then join a church based NGO mission there's loads aroung, although you have to have some relevant skills or something to offer. If it's just to help others in need try see if there some voluntary or peace corps type of assignments. Other than that get a masters in business, law and/or international development and try joining the big institutional gameplay.

Sadly it's not easy to just travel and get involved these days..it should be, but the world doesn't operate like that anymore, to many border and laws...but still worth ago if you are keen to try..as they'll only kick you back home again.