I need some help with improving vocals?

I guess I should say "starting" vocals. Sure, anyone can sing, but I wanna learn to scream - both low and high growls. I am a big fan of most heavy metal stuff, especially bands like Whitechapel and As Blood Runs Black. However, in terms of learning vocals, I want to have a similar style as George Fisher, the vocalist from Cannibal Corpse. He is my idol for growling, but I really suck at doing it. I don't have necessarily a very "deep" voice to begin with. It's not high, but it's not really low. However, a clmate in my old school had a really high voice and yet he had the ability to be the screamer of his band. I don't know how to do it though. Help?