http://www.nasaproracing.com
http://www.scca.org
Easiest and cheapest way I've seen to get started into motorsports is to do solo 2 auto-x with the SCCA (sports car club of america).
A lot of people (myself included) skip that and start out with the HPDE's(high performance driving events/education) by NASA (national autosports association)
http://www.speedtrialusa.com
and
http://www.carguysinc.com
also host HPDE-like events
Lot's of fun
here are some random links to help you on your way...
http://www.turnfast.com
http://www.solo2.org
http://www.autocross.com
http://www.autox4u.com