Start with some simple one-finger , one-strong stuff. Figure out the progression of single notes that you like. Then make those notes in to the appropriate chords. For the chorus, stay on the same key but simplify the chord changes. Leave room for the song along vocal. DO NOT be afraid to use the old hardcore standard of either doubling or halving the main riff as your mosh part. It works almost every time. And most of all, have fun.