When you became a certified Skywarn storm spotter, you should have gotten a phone number to call to report severe weather. Calling them is one of the best methods to report and event. There is also
http://espotter.weather.gov/ which is an online report system. Though it is not an official Skywarn website, an alternative service is
http://spotternetwork.org/. Some NWS WFOs are also integrating social networks like Facebook and Twitter into their reporting systems as well.