Ok Threadstarter, first of all. You are 14. You have to let your balls drop a little first before you can acquire a raspy voice.
As far as screams go, go to musicianforums.com and read up on screaming/growling. There you will find information on how to do that stuff. It also takes practice. I've been screaming/growling for about a year or so now, and I've developed it so it doesn't hurt, and I can add a lot of rasp to my voice without damaging it. Remember, DON'T YELL!! If your throat starts hurting, stop or you will fuck up your throat.
How I learned is I basically listened to artists and tried to emulate the sound they produced(and I also learned from Musicianforums). As time goes by, you will be able to push harder and make your voice stronger. For now, you are young, and trying to accomplish a technique like this can be hard. For me, i'm still trying to perfect my technique and get better at it, and it took me well over a year.