How do bands write songs?
One of the most common ways to write as a band is to have a single songwriter that writes the music and presents it to the rest of the band, acting as a critique group which gives feedback and suggestions to make the song better.
Do bands read sheet music?
As Borodog stated, very few rock bands use, or read, standard notation. Some of the more technically accomplished bands, like Rush perhaps, might, but otherwise, can't imagine sheet music being necessary.
How do metal bands make songs?
A metal song, like rock and pop, is usually made up of a combination of verse, chorus, and bridge, with an optional intro and outro. You'll need to work with your rhythm guitarist to create a structure based around the original riff or melody you came up with.