Typically, when people think of minor scales, they first think of natural minor scales. Natural minor scales are probably the most common minor scales out of the 3 different minor types and are sometimes referred to as the default minor scale. Compared to a major scale, the natural minor scale lowers the 3rd, 6th, and 7th scale degree of a major scale by a half step. For example, F Major consists of the notes F- G - A - Bb- C - D - E - F. To change the scale to be in natural minor, lower the third A to an Ab, lower the sixth D to Db, and lower the seventh E to Eb. So, F natural minor scale would be F - G - Ab - Bb - C - Db - Eb - F.
Harmonic minor scales look very similar to natural minor scales. The only difference between these two types of scales revolves around the 7th scale degree. To make a harmonic minor scale, first take the natural minor scale and raise the 7th scale degree by a half step. For example, to change F natural minor scale F - G - Ab - Bb - C - Db - Eb - F to harmonic minor, the 7th scale degree Eb would be raised a half step to E. So, F harmonic minor scale would be F - G - Ab - Bb - C - Db - E - F.
The notes in Melodic minor scales change depending on whether it is ascending or descending. The only notes that would be changing in this type of scale are the 6th and 7th scale degree, When the notes are ascending or going up, melodic minor scales are very similar to their major scale form with the exception of having a flat 3rd (the 3rd is lowered by a half step). In other words, when ascending, melodic minor scales only contain a flat 3rd. For example, F major is F- G - A - Bb- C - D - E - F. To change this scale to become melodic minor scale when ascending, lower the 3rd A by a half step to become Ab. When the notes are descending or going down, lower the 6th and 7th scale degrees down, so in total the 3rd, 6th, and 7th scale degrees are lowered by a half step during descend. In other words, melodic minor scales look exactly the same as their natural minor form when descending. For example, F natural minor scale going down and F melodic minor scale going down would look like F - Eb - Db- C - Bb - A - G - F.