In reply to Andy S:
No, Americans generally have the teaching in Africa side cornered. Teaching English does not mean you can earn enough to even survive. You still need to back yourself to some extent. Africa is not a good place to start, everyone wants to help Africa and so much is not even volunteer work, you pay for the honour of helping.
True voluntary work is NOT available to those with no skills to offer, end of.
Think outside of Africa to start with, then you will network with others who live the voluntary scene and will point you to where is coming up and where is a dead cert, where is paid and where is F&L.
Alternatively look into HELPX.com, you will find endless contacts throughout the world, you just got to get there and your trip begins.