ii Minor - D minor

In the key of C, D minor is the chord that is built on the second degree of the major scale. Since the notes of the chord make up a minor chord, this chord is known as ii minor (reads 2 minor).
The notes in the Dm chord are D, F, and A. The interval between D and F is a minor 3rd (3 semi-tones) which gives the chord it's characteristic sound.
Strum the chord and sing the notes that make up the chord.
Then practice alternating between C and Dm.

C | C | Dm | Dm

Sing the roots as you go.
Hear the difference in the chord quality between the two chords?

iii Minor - E minor

E minor is the chord that is built upon the third degree of the C major scale. Em is the iii minor chord in the key of C.
The notes of Em are E, G, and B. Again, like Dm, the interval between E and G is a minor third.
Strum the chord and sing the chord tones.
Then practice switching between C and Em.

C | C | Em | Em |

vi Minor - A minor

In the key of C, A minor is the chord that is built on the sixth degree of the major scale.
This is known as the vi minor chord as well as the relative minor.
The notes in Am are A, C, and E. The interval between A and C is a minor 3rd (3 semi-tones) which gives the chord it's minor quality.
Strum the chord and sing the notes that make up the chord.
Then practice alternating between C and Am.

C | C | Am | Am

Sing the roots as you go.