How to adopt a baby from outside the U.S.?

I am Iranian and although I now live with my wife in San Diego, I still have many family and friends in Iran. A good friend’s daughter is pregnant and very young and unmarried. It is shameful for her to be in this state given my friend’s religion and the culture in his community. My friend knows my wife and I have been trying to have a child for years and he has offered for us to adopt the baby. His daughter is agreeable. What do my wife and I need to do to adopt the baby and get it here to the U.S.? Do we need a lawyer to do this or do we work through some sort of agency? Will the child automatically be a U.S. citizen once we adopt?