I'm not an instructor and this may not be correct but when I write a song it usually starts with a little jingle that I sing and then try to find the key on the guitar.

So I would start by just singing out the lyrics. Say the chorus. Try to find a note that seems to sounds good with what you are singing. There will be a note that predominantly fits. Let's say E sems to work pretty well. Try an E major chord and then an E minor chord. Which sounds right? That should work to establish your key. Then put the chords together and strum pattern.

That's what I do. But I'm not writing for Celine Dion, just stuff for a few of us to play along to.