This website is based in the UK but I seem to detect US influences in the wording of your question. It might make a difference as, for example, there would be no point in us suggesting that you contact a UK-based charity if you're in the USA.
So some more information might help here. e.g. where are you based, have you got any particular specialist skills and how long do want to devote to a medical mission?
However, to get you started (wherever you are), take a look at the MSF website:
http://www.msf.org/en/work-msf