The relative minor for C is A minor because notes for the scale are the same. To find the relative minor for a key you go down a third. A third down from C is A so you play the same notes of the C scale but the tonic is A.

You do this with every key, keeping in mind the accidentals. For instance the key of G has one accidental, F#. The relative minor of G is a third down which is E. Then you play the G scale but the tonic is E. That's how you know the notes of the E minor scale. You start on E but you're playing the G scale.

Another way to look at it is to reverse it. Say that you know what your tonic is and you're trying to do a minor scale. Let's say you want to play a C minor scale. Your tonic is C but the scale is Eb, which has three flats. Eb is a third up from C. So you play the Eb scale, but you start on C. Your C minor scale is C,D,Eb,F,G,Ab,Bb. Remember a third is three half steps.

The easiest way to play a minor scale on the guitar is by using the Aeolian pattern with the same tonic of your minor chord, Bm is B tonic.