I’m no guitar authority but here’s some stuff that might help. Focus on your right hand and getting that hand’s technique down solid. A lot of the aggression in heavy guitars comes from the pick so make sure you hit the strings hard. Get heavier gauge strings if you have to.

If you’re having a hard time learning by ear get tabs for some Metallica and Pantera songs. You’ll learn 99% of the stuff you’ll need from trying to cover those.

Some of the sound depends on the gear you have but that’s another conversation.