Play the A chord play from A to A 5th fret Low E to A note 7th fret D string. 4th fret G string. You can play the Am scales and major scales over the A5. The third note of the chord makes it a minor or major without it, it would fit over both scales. The Major key is just a starting point.
Every song needs a tonic note.
So you need to determine if it is going to be based in the major or minor scale. Either based in Aeolian or Ionian. The minor scale has a weak cadence, while the Major is stronger one. The melodic minor is okay, but sounds to much like the major scale and doesn't sound to good to use all the time. It is often used to help the harmonic minor.