Right screaming/growling comes from the right inspirations (like everything in music)
It matters how old are you and how long you've been practicing...you need firtstly to learn how to "damage" your throat
Then as the time passes you will becoming more brutal and growly...This video may help you...the best technique in my oppinion (tested by me)
http://www.youtube.com/watch?v=jb6o2duqE7o