Write down all of the chords.
Write down the scales associated with each chord. i.e: If you have E minor, write down the E minor scale. ...
Look at each scale and see if the chords' root notes are within that scale. If they are, that is the key you are in.
Write down the melody first. from there, look at all the possible chords that could go underneath. try to pick out the outermost notes in the chords. from there, it generally follows that you can figure out the chord structure. I can't listen to the song right now or else I would be further help.