Learn Music Theory:: Finding Key Signature & Flat Harmonic Minor Scales

Harmonic minor scales have a special pattern between the notes like the major scales had.
That pattern is: Whole step Half step Whole step Whole step Half step Whole and a half step Half step
These steps are called intervals. For harmonic minor scales the raising of the 7th note applies when the scale is ascending and descending.

When you are working with minor scales you need to find the corresponding key signature from the major scales. When you are give a minor scale you take the letter name of it and either count three notes up or six notes below to find the matching key signature.
If you are given A minor, in order to find the key signature you'll count three notes up: A-B-C and C is the corresponding major so you take the key signature from C major which is no sharps and no flats so that's the key signature for A minor.
Another way to do it is going A-G-F-E-D-C. I recommend using the first way since it's faster and easier to do but what ever works for you is what matters.
When counting the notes you must remember to include the first letter of the minor scale otherwise it'll be all wrong.

Another example: D minor.

D-E-F F Major has 1 flat: Bb so you take this key signature and apply it to D minor.
or
D-C-B-A-G-F

Familiarizing yourself with this method will help a lot with learning the harmonic minor scales.
A harmonic minor scale is the same as the natural minor but raising the 7th note.

A minor has no sharps or flats but A harmonic minor has the B raised.
When you raise the seventh note you raise it by half a step. So you would raise B to B-sharp which is the same as C since there is no note between those two.

a harmonic minor looks like: