How to Belt Or At Least Sing Very Loud?

http://www.youtube.com/watch?v=uVZIyRQMVg0&feature=youtube_gdata_player there's me singing ( I'm doing elphie's part ) i'm not completely sure if i'm using mixed or head voice, but that's completely irrelevant. i want to learn how to belt, like teal wicks, idina menzel, and stephanie j. block do in defying gravity. i know it's a complicated subject and it takes years to study, but could someone please point me in the right direction?