Every major key has a relative minor, and every minor key has a relative major. These share a key signature, not a name. For example C major and C minor are completely different. C major has no sharps and no flats, while C minor has three flats.
To find the relative minor of a major key:
- Write the root note of the major key on a stave, then write the third below. This gets you the letter name of the minor key
- Count down four semitones (using the original note as 1) from the major key to find the chromatic name of the minor key
It is very important to follow both of these steps, because if you miss one out it is very possible to get the minor wrong. For example, if you were trying to find the relative minor of A major and only followed step 1, you would get F minor. If you only followed step 2 you would get G♭ minor. If you follow both steps, you will know that the relative minor of A major is F♯ minor.