a minor and c minor scales

why does a minor scale start on a minor, but c minor starts ok c major?

Generally the scale starts on the tonic (name of the scale, C for example).

But sometimes when teaching the relationship between minor and major scales, the instructor will start on the same note to help visualize the scale pattern. so in this case the scale wouldn't start on the tonic. I'm not sure if this answers your question or not?

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.