Before you do anything else, buy a copy of John A. and Alan Lomaxes Folk Songs:USA--a wonderfully entertaining and well written book that gives you plenty of material to work with and plenty of the real sense of the lives that people lived and they way that songs reflected them--

it is also important to realize that everything else that you may find that uses folksongs as a way to focus on understanding lives, society, and history, including even the excellent Contemplator web site, follow in the footsteps of their work(in a pinch, you could probably do everything that you wanted to do by just using these two sources, but that would be a bit shakey, and anyway, it wouldn't be as much fun as just using them as a starting point)--

Next thing is for you to make up you lesson plans, then ask people to post songs that relate to each of the topics that you will be teaching, rather than getting a million songs about every historical event imaginable and trying to make build a curiculuum--You don't want to end up saying"Here's another song about...something historical."

Anyway, many have paved the way for you, and with a little library research, you can find some excellent resources--