It really depends on your preference. Really, sometimes I'll just geta nice good chord progression out of a key, or sometimes I'll just hear something in my head and get it by ear. And A7 and dsus, D/F#, they're just chord variations. They come into play whenever you want em to. If you want to have a 7 or Maj7 chord in your song, that's fine, go ahead, variety is good. It's really all theory though, to figure out chords like A7, dsus, and D/F#(that ones pretty explanitory). Like for example, A7 is an a chords but you're playing an open g note. The reason is is because your saying play the 7, but since it's not a Maj/or7 (which is the 7th in the Major scale) you flat the 7th, so the 7th of A, is G#, and ifyou flat g# you get G. So that's how you get A7. D/F# just means play F# as the bass. Dsus, I forget what sus's are? Aren't they where like you add. Like wouldn't Csus4 be like playing a C but with an F on the d string. I don't remember about the sus, so just look it up. But someone can confirm my "idea" i guess with sus's. I mean, practice is all it takes. I remember writing songs, and they just get better as you go along. Trust me, you won't get good at it at first. But don't forget soul, you need soul. There's a song, all it is the 1 and 4, d -> g, over and over which some other chords in the chorus, but it's such an awesome song cuz it has soul. And I think going by ear is a good way to just start writing songs, cuz at first I had lots of good tunes, then they got more bland, so I have to start hearing more melodies than real tunes, and kind of variate em on the guitar so they're not so "bleh". So I suggest just getting all those good ones out first. And intervals, ask about em here, are a very good way to write songs.