The natural minor scales use the notes from the key signature. Every natural minor has a relative major scale with exactly the same notes. For example, A minor and C major have the same seven notes. The sixth note of the major scale is always the tonic of the relative minor scale. Db major and Bb minor, D major and B minor, etc. So if you know all of the major scales, you can easily learn their relative minors (but the traditional fingerings are different since we are starting on a different note).
Another way to compare major scales to minors is to learn the relationship between the scales that have the same tonic. We call these the parallel major and minor. C major and C minor for example. The difference between these scales is that we add three flats or make three sharps natural (or some combination). Since C major has 0 flats and 0 sharps, C minor has 3 flats. Since D major has 2 sharps, D minor has 1 flat.
It turns out that the notes that get lowered in the minor scale are always the same notes of the scale. It is the third, the sixth, and the seventh. Those are the notes that are different in the natural minor from the major. However, in actual music it doesn't always work that way. For example, we often have a cadence in C minor that goes G7 to Cm. G7 has a B natural, not a Bb. B natural is the leading tone that is thought to pull us back to C. So that is why harmonic minor scale is taught, to emphasize this raised seventh degree which is often used in minor harmonies. Therefor the harmonic minor is the same as the natural minor, except the seventh scale degree is raised back up. In C minor the harmonic scale has an Eb and an Ab, but the B is made natural agin.
When you play the harmonic minor scales, many people feel that it sounds exotic or foreign. This is because the scale has created an unusual interval between the sixth scale degree and the raised seventh. It's an augmented second, and it sounds kind of weird when we play it, as well as creating awkward fingerings! Well it turns out that in the traditions of Western music, melodies weren't supposed to use that interval. So if they were approaching the tonic from below, and we raised the seventh scale degree to create the "leading tone", we would also raise the sixth scale degree to avoid that weird interval. This is the meaning of the melodic minor scale. When we go up, we raise the sixth and seventh scale degree to simulate how it would be used in music. When we go down, we change them back to the way they appear in the key signature.
The way I learned melodic minors was to think of the ascending version as being almost a major scale. Only it has the minor third. So C minor, the ascending melodic minor has just Eb and everything else is the same as the major. But then going down, you must remember to change that B and A back to Bb and Ab. You just need to remember it is the sixth and seventh notes of the scale that have to change.
Topic: minor scales