Before you even try to learn to scream you need to figure out which kind of scream you want. The two that I can teach are the False Chord scream and the Fry scream. If you don't want to do either of these, then you will have to look elsewhere for help. Now if you don't know the difference between the two, I have uploaded songs that show what each sound like in the right hand side bar.
There are these muscles called Thyroarytenoid muscles (which is just fancy talk for your true and false vocal chords). Your False Chords make up the upper half and your true vocal chords make up the lower half of the Thyroarytenoid muscles. The False Chord scream is produced when the false vocal cords press together and muffle the true vocal cords via air from the diaphragm.
The Fry scream is produced when your upper Thyroarytenoid muscles (true vocal chords) are stopped from vibrating and a continuous amount of small air is pushed through them via the diaphragm.
False Chord Scream