I'd think it's really best to lay aside the guitar or whatever until the song has reached a singable stage, verse and chorus, if you've got a chorus.

Then it's time maybe to work out what kind of accompaniment it needs, and which chords work best.

Generally speaking I find the chorus grows naturally out of the verse. Sometimes it's just one of the verses, with the same tune, sometimes with a modified tune, sometimes a couple of lines that started out as a verse.