For the second question - the song is in D scale. There are a set of notes only which u use in that scale. So you also choose chords which have those notes in it. D major Scale has these notes- D, E, F#, G, A, B, C#. So if I use D chord( D, F#, A) these notes are in the scale so I can use this chord. Similarly G Chord- G, B, D and A chord- A, C#, E. Hope it answers ur question. Better don't bother about these technicalities. Just check out the chart I have given which gives what all chords to use for any scale. I didn't get ur first question.
