Before you begin take a look at how a major scale is constructed
Look at the C-major scale
C D E F G A B C
Find the sixth note. (The submediant). That note is ‘A’.
Now write a scale, starting on the ‘A‘,and proceed upwards for one octave.
A B C D E F G A
Now if you have already looked at how major scales are created you should know that Cmajor has no sharps or flats. This is the same for Aminor which has the same key signature as it is the relative minor of Cmajor.
So to find the relative minor of a major scale, find the sixth note of the major. That note is the note upon which the relative minor would be built.
The type of minor scale you just learned to construct is called a natural minor scale. Sometimes you may see them referred to as “Pure minor“.
There are two other types of minor scales you need to learn: the harmonic minor and melodic minor.
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HARMONIC MINOR:
To form a harmonic minor scale, take the natural minor, and raise the seventh note. To change the A-minor scale above into a harmonic minor scale, we would raise the seventh note, the ‘G‘ to become a ‘G#’. Here it is:
A B C D E F G# A
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MELODIC MINOR:
To form a melodic minor scale, take the natural minor, raise the sixth AND seventh note on the way up, and put them back to their “natural” state going down. Because the melodic minor looks different going up than it does going down, you must write a melodic minor ascending and descending. Here is a A Melodic minor scale:
A B C D E F# G# A G F E D C B A
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You should learn all these types of minor scale.
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Determining Key Signatures of Minor Keys
Hopefully you should know how to use a major scale, find the note upon which the relative minor scale will be constructed, and write the three different types of minor scale.
But what happens when you are asked to write the key signature of Aminor ?
As you know, a minor scale and its relative major will share the same key signature. In the case of A-minor, you know that ‘A’ is the sixth note of Cmajor scale. Simply go up a whole tone, plus a semitone. This will get you the relative major. So a whole tone plus a semitone above ‘A’ is ‘C’. C-major will use the same key signature as A-minor.
If you’ve understood the page dedicated to major scale construction you will already know that C-major has no sharps or flats. So now you also know that A-minor has no sharps or flats as well.