Find a metal voice teacher or a vocal coach who has an understanding of the techniques used to scream. A traditional vocal teacher may not care to teach screaming techniques seriously, but there are two highly acclaimed online instructional methods that will guide you (see resources).
Restrain yourself. This sounds like the exact opposite of what you want to do, but screaming takes vocal control. Start with simple scale lessons in the range you're most comfortable with. Try singing one of the scales that come with your lesson plan. Sing along and match your pitch to what you hear on the CD. Find the lowest note you can sing comfortably and the highest. Those will be the notes you sing again and again. After you've practiced this scale several times, try to push yourself to start one note lower and go one note higher. This is stretching your vocal range.
Record your practice sessions and play them back. Listen critically. Are you singing each note clearly and in tune? Be sure you can hear the power behind the note. If you sound weak, you need to work on singing from your diaphragm rather than your throat. Singing from your throat will not give you the power you need and it will damage your vocal chords in the long run.
Practice breathing. You vocal course will discuss breathing in depth. You need the proper breath to push a scream out. The combination of singing from the diaphragm and proper breathing is what will release your screams.
Try building your screams when you've reached a vocal range of at least an octave and a half. Start low in your abdomen and let the scream begin as a rumble. Let it build naturally and explode from deep inside you. Once the scream is in motion, try vibrating your vocal chords to enhance the scream with vibrato. Work on changing the pitch of the scream as you hold it out. Most importantly, stick with the method of the vocal course you choose, and soon you will be screaming like a banshee.