Just like any genre of music there are LOTS of sub categories.. there are MANY types of metal.. what kind of metal do you like? (what bands) Most Metal acts write MOSTLY in a particular vein.. Metal is typically about anger, agresion, sex, rebellion, politics, religion, etc. etc. etc. You can write about whatever moves you, but If you want to write Metal think about HOW the music makes you feel.. the energy.. the mood... etc. etc. the... topics that stir those same emotions in you are probably the best to cover in your metal lyrics... it's good to have the music re-enforce the lyrics... and vice versa.

good luck with your Metal.. I look forward to seeing some metal lyrics posted on the boards!