The circle of fifths is an organized way to try and remember key signatures. 

This is the circle of fifths for major keys.

All scales are determined by the intervals from note to note.  For example, a minor scale has half steps between 2 - 3 and 5- 6.

The following example shows a basic A minor scale.

W = Whole step

H = Half step

Remember - a tetrachord is a four note scale pattern.  Two tetrachords combined make a scale.  In a minor scale, only the lower tetrachord is the same.

Lower Tetrachord

1 - W - 2 - H - 3 - W - 4

Upper Tetrachord

5 - H - 6 - W - 7 - W - 8

Together

1 - W - 2 - H - 3 - W - 4 - W - 5 - H - 6 - W - 7 - W - 8

With minor scales there are also variations.  They are:

Harmonic Minor
Melodic Minor (Ascending)
Melodic Minor (Descending)

The following are the tetrachords:

Harmonic Minor

Lower Tetrachord

1 - W - 2 - H - 3 - W - 4

Upper Tetrachord

5 - H - 6 - W/H - 7 - H - 8

Together

1 - W - 2 - H - 3 - W - 4 - W - 5 - H - 6 - W/H - 7 - H - 8

Melodic Minor (Ascending)

Lower Tetrachord

1 - W - 2 - H - 3 - W - 4

Upper Tetrachord

5 - W - 6 - W - 7 - H - 8

Together

1 - W - 2 - H - 3 - W - 4 - W - 5 - W - 6 - W - 7 - H - 8

Melodic Minor (Descending)

Lower Tetrachord

8 - W - 7 - W - 6 - H - 5

Upper Tetrachord

4 - W - 3 - H - 2 - W -1

Together

8 - W - 7 - W - 6 - H - 5 - W - 4 - W - 3 - H - 2 - W - 1

All scales are determined by the intervals from note to note.  For example, a major scale has half steps between 3 - 4 and 7- 8.

The following example shows a basic C major scale.

W = Whole step

H = Half step

Remember - a tetrachord is a four note scale pattern.  Two tetrachords combined make a scale.  In a major scale the tetrachords have the same intervals.  The lower tetrachord and upper tetrachordhave the same intervals.

Lower Tetrachord

1 - W - 2 - W - 3 - H - 4

Upper Tetrachord

5 - W - 6 - W - 7 - H - 8

Together

1 - W - 2 - W - 3 - H - 4 - W - 5 - W - 6 - W - 7 - H - 8

The names of the following scales come from something called church modes.  Through modification over the years these scales eventually grew into major and minor scales.  Much of this can be heard in "chants" or gregorian chant.

You may notice that two of these scales will stand out tonally.  Aeolian and Ionian.  Play them out and listen to how they relate to what is used today in your composition.