For understanding the background to what the circle of 5ths is, I recommend reading the Wikipedia article: https://en.wikipedia.org/wiki/Circle_of_fifths

In short, this circle helps us with remembering the number of sharps and flats in the major and minor scale and which notes they are. I can warmly recommend taking a piece of paper and drawing this on it.

The notes in the C major scale are precisely the same as in the A minor scale, yet you start and end on a different note. You will hear that this entirely changes the character of what you hear. When learning a scale, you also want to learn how to play that scale while starting on any note of the scale. This is called, playing different degrees of the scale. This means that starting to play C major scale on the sixth note of the scale is the same as playing an A minor scale. To understand modes, I recommend reading the Wikipedia article: https://en.wikipedia.org/wiki/Mode_(music). We will use the modes as described under “Modern Modes” or “Modern Western modes”.