The way to form a Minor Chord is to take a Major Chord and flatten the third.
Form a Major Chord, make sure you know which note is the third and move it down by a semi-tone (1 fret).
So a Minor Chord is Root, 3rd flattened and 5th (plus any of the notes repeated).
Example: G minor
because the scale of G is:
In theory then, just moving one finger down one fret turns a Major chord into a Minor chord!