Each diatonic chord is made up of three notes within its scale: a tonic, a third, and a fifth. For those of you who don’t read music, the notes in the C triad above are C (tonic), E (third), and G (fifth). The second note in the C major scale is D, which is assigned to D minor (D as tonic, F as third, and A as fifth). The third note is E and is assigned to E minor (notes are E, G, and B).
In short, the second, third, and sixth notes in the C major scale (D, E, and A) are minor chords, while the tonic, fourth, and fifth notes (C, F, and G) are major chords. The seventh note (B) is a diminished chord, which means it is comprised of a tonic (B), a minor third (D), and a flat fifth (F).
Relative major/minor keys
For every scale there is a relative major or minor scale that shares the same notes. To find the relative minor scale to a major scale, start at the tonic and count a half step and a whole step down. In the case of C major, the relative minor key is A minor (C to B is a half step, B to A is a whole step). To find the relative major of a minor scale, start at the tonic and count a whole step and a half step up (A to B is a whole step. B to C is a half step). The diagram below shows the relationship between a C major scale on the top and an A minor scale on the bottom.