I keep asking James (who runs Soft) to start up a forum on songwriting but alas to no avail.
Anyways - the key should be decided early on cos' you're singing the song and you would have your range set out from the word go.
Lyrics or shords first ot a riff to give you the start you need - it doesn't matter. They all work. It's an artform dude! If it works, then why bother with one way or the other - or another?
WHen you're taking about chord families, I am assuming that you are talking about KEYs. Well, there are musical formulae for doing that and theory is involved - that's beyond me.
But nothing beats hearing the stuff in your ear and you ease in and out of keys as though they never existed - almost chromatic - priceless when that happens!
My advice, just go with the flow. Whatever happens - just let it flow. Songwriting is a process. And good songwriting requires experience and maturity. Start as soon as you can. Work towards quality and not quantity and you should be fine.
The rest of the way - well, that requires talent. And if you're hard at it, hopefully in due time we'll hear your stuff.
Bestest
RoRK