Depends on what you want to do,

NRA http://home.nra.org/#/home

IPSC http://www.ipsc.org/

IDPA http://www.idpa.com/

Steel Challenge http://steelchallenge.com/

Benchrest http://www.benchrest.com/


Appleseed (Not really competitive shooting, is CMP affiliated, teaches Basic Marksmanship while giving a History Lesson) http://www.appleseedinfo.org/

Airgun Arena has a Air Rifle EMatch http://www.airgunarena.com/index.php/Main_Page