Hi Robin-
I've never done this before, but one of the things you should do first is work with your local government (town / city) to check into ordinances that list requirements for a NFP organization.
A close friend of mine has established one and it took years, including legal set-up and assistance from a tax accountant to get going. Depending on the type of work you are planning on doing, some of this will be more important than others. Some of the work involved includes registration of the group, website creation, fundrainsing plans, and establishing a board of directors.
You'll also need to determine if you want to be a lobbyist in your State for your cause (totally depends on what you want to do).
A good on-line resource is
HERE. Of course, the government has a great site that can help
HERE.
I hope that helps!
Thanks,
Lisa