Relative or Natural Minor
In order to invoke the Relative or Natural Minor of a scale, simply begin at the sixth degree and follow the above interval formula of tone, tone, semitone, {tone}, tone, tone, semitone. Therefore, the sixth degree (note) of a C major scale will yield a Relative Minor of Am. Of course, as with the major scales, the minor scales are divided into two tetrachords. As one might incidentally observe that starting on the beginning of the upper tetrachord or 5th degree, you effect the next minor scale around a pattern similar to the Circle of Fifths. This is sometimes referred to as the Circle of Fourths.
C Major with relative A minor
Harmonic and Melodic Minor
To form a harmonic minor, simply raise the seventh degree of a natural minor (A, B, C, D, E, F, G♯). Likewise, to yield a melodic minor, the seventh and sixth degrees are raised (A, B, C, D, E, F♯, G♯).
A harmonic minor
A melodic minor