Keeping the theory as simple as I can, a major chord is the 1st, 3rd and 5th note of a Major scale. The best way to start with music theory is to use numbers. There are 7 notes in a Major scale = 1 2 3 4 5 6 7 1(8)
8 being the starting note again an octave higher
If we talk in intervals (distance between two notes) we can expand this a little further but it’s easier to think in plain numbers at the start.
1 = Unison
2 = Major 2nd
3 = Major 3rd
4 = Perfect 4th
5 = Perfect 5th
7 = Major 7th
8/1 = Perfect Octave
So again a Major chord is 1 3 5, If we take the 3 and lower it by 1 semitone* it becomes a b3 or a minor 3rd. This makes the chord minor.
*A semitone is the closest note to another note, a 1 fret movement on the guitar.
Major chords have a “bright” or happy sound
Minor chords have a “darker” or sad sound
Major chords are written with just the letter so A = A Major sometimes people write it with a capital M so CM = C major
Minor chord are written with a lowercase m after the chord so Am = A minor
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