You don't have to start a new organization; just look around for those that already exist.

American Atheists
American Humanist Association
Americans United for Separation of Church and State
Atheist Alliance
Council for Secular Humanism
Freedom From Religion Foundation
International Humanist and Ethical Union
NoBeliefs.com (Freethinkers)
Secular Web: Atheism, Agnosticism, Naturalism, Skepticism and Secularism
The Brights

Hint: Go to one of these web pages and then use Google to find "similar pages." It's really easy with the Google Toolbar.

Aternative method: Use something like this as your search argument in Google:
related:http://www.secularhumanism.org/