The most widely recognized national organization for radio amateurs in the US is the American Radio Relay League, or ARRL. The ARRL is well known for producing a monthly magazine called QST, which contains technical articles, regulatory updates, contest calendars, list of special operating events, and other things of interest to amateur radio operators. In additon, the ARRL serves as ham radio's advocate in Washington DC, keeing an eye on developments in the FCC and in Congress. It also happens to be the largest Volunteer Examiner Coordinator (VEC) in the country.
The ARRL maintains a web site that is full of information for hams at: http://www.arrl.org. If you look around and like what you see, you can join by going to http://www.arrl.org/membership .