My amatuer band also composes metal. we are just starting out. but when composing, firstly, think of whether the song is instrumental or not.
if looking for lyrics, try starting with a general topic and slowly moving to a more pin ponted objective in your lyrics. or, you could just say random words and link them. haha:D
for the chords, try to down tune you guitar(you should have done that alr) you can go as low as you want. haha, for the chords, since you are a classical player, try some of Yngwie Malmsteen's riff or get inspiration from them. his riffs are neo-classical but work real well with heavier music too!
try minors too. lets say you play d. to make it sound more dark, do a e flat. and add in some sweeps like combining the d and e flat arpeggios. pentatonics can also come in handy when out of ideas. try listening to paul gilbert for some inspiration. get some tips from his vids in youtube.
all the best!:)
Zaob
