So like it says I need help to try to write a Post-Thrash riff. I am not asking anyone to write one for me, I just need to know how to write one. What scales and chords should I learn on guitar? I keep listening to bands such as Machine Head and Pantera and I got no inspiration.

Okay, there are way too many sub-genres of metal out there. What the hell is post-thrash? Pantera and Machine Head are groove metal.

If you want to write music in a certain style, don't just listen to it, but learn how to play as much of it as you can. After playing it for a while, it will affect your style of playing/writing.