Well it depends, quite often it starts with a bass line, I'll know what genre I'm after and just noodle for a hook, then brief outline of a song structure comes next, choosing the key to suit my (or another's vocal range) then lyrics are formed, basic instruments then follow and a guide Vocal, all the other instruments come later, maybe then Lead Vocal with usually thing like percussion and hooky fills last.
It used to be piano that I'd write my songs, but not so much now, although I had a couple recently that emerged this way.