Tunes in a minor key are determined by the key signature for the music. For example if a tune is in the key of C major, there are no sharps or flats (the black notes on a piano). A tune in the key of C minor however has the third note flattened (E Flat) and the sixth note and seventh note (A & B flat). The key signature will therefore show three flats (B, E and A - all black notes on the piano).
The following link has more information
https://en.wikipedia.org/wiki/Key_signature
Hope this makes some sense!!