While it's true that any and all ways can work, I think you work from the center out. Meaning, "Smoke on the Water" started with a riff, "Yesterday" began with a melody, "Born this Way" began with a bacon suit, lots of songs begin with beats these days. You get the idea. For me a few songs came from a guitar tuning. But know that's what your song is going to be, in my case a song to highlight how friggin cool that tuning is and how excited I was playing around in it.
But the hook element of the song is where you usually start, what's going to define the song. Usually the element is a vocal line or lines, but not always. (For most listeners, obviously, a great line or melody is alot easier to grab onto than a cool tuning)
I used to write alot of songs beginning with a "message" or philosophical point I was trying to make. Most of them ran out of steam after a verse or two.
Just my angle.