Fist off you have to be able to enjoy horrible music...and be angry about political/religous/other forms of teen anger. You have to be able to run around like a complete fool and have the people who know how to play real instruments laugh at you...!
you have to breathe in and then dont scream from your throat like you would yell at someone you have to scream from your stomach, like push it out from your stomach. for low screams make your mouth smaller and for higher screams open your mouth wider. It takes practice..Ive been practicing and im pretty good...good luck, i hope i helped.