basically, in a song, the 1st, 4th and 5ths chords are majors. the rest are minors.
take for example Earth Angel (key of C)
C, Am, F, G
in this case, the chord Am (6th) is a minor. basically because it contains the notes in the key of C major (C, D, E, F, G, A, B, C)
a chord is basically made of the 1st, 3rd and 5th (eg C major C, E, G. refer to the letters above in the C major scale)
Am - A, C, E (in a minor chord, the 3rd is flatted)
if it was A major chord - A, C#, E, it wouldn't fit because C# is not in the C major scale.
so for the rest of the chords, if u work it out, they are all in the C major scale.
however, there are exceptions to this rule.
eg Don't Look Back in Anger - Oasis
Basic Chords (key of G)
G, B, C, D
here, the 3rd is in major. dun ask me why. but it juz seems to fit in with the song.
the limitations to music are endless. dun let my points tie u down in writing songs, but use it as a starting guide.
PS. u dun really have to look at the circle of fifths[/b]