Melody/lyrics, chords, bassline, drums. That's how you write a lead sheet. You have to know what you want to say in order to know how to sing it, you need melody to find chords that fit it, you have to know the chords to write a bassline that won't clash, and then you set a rhythm to the whole thing and that's where drums come in. You can do chords first to make sure the melody you're writing doesn't harmonize into impossible chord progressions, and also to use that cool progression you found as a fill, but IMO it's best to have the first three before thinking about the last two.