The Circle of Fifths is a very important concept in music theory.  Once understanding the Circle of Fifths it will be much easier for you to find relative keys and key signatures.

Relative keys:  A major and minor scale that have the same key signature(notes) are said to be in a relative relationship.  To find the relative minor of a major scale go to sixth degree of the major scale and that will be the relative minor.

C(1st degree) – D(2nd degree) – E(3rd degree) – F(4th degree) – G(5th degree) – A(6th degree) – B(7th degree) – C

As you can see A would be the relative minor of C because it is the 6th degree of C, if you were to extend this C to C would be a C Major scale and A to A would be an A minor Scale.

C – D – E – F – G – A – B – C
C Major Scale
………………………………A – B – C – D – E – F – G – A
………………………………A minor scale

To find the relative Major Scale of a minor scale count to the third degree of the minor scale and that note will be the tonic of the relative major.

A – B – C – D – E – F – G – A
A minor Scale
…………..C – D – E – F – G – A – B – C
…………..C Major Scale

The Circle of Fifths is a visual representation of this as it can start to get difficult or just annoying to try to figure this out if you are composing a lot of music, doing a lot of music theory homework or working with keys that have several sharps or flats.

Circle of Fifths