Have you been introduced to the Circle of 5ths? If not.....
Key of C at the top.
It's relative minor key is inside the circle, i.e. the A or Am scale. Let's take a deeper look.
Notes for the C scale are; C, D, E, F, G, A, B
Notes for the Am scale are...................A, B, C, D, E, F, G
Chords from the C scale are C, Dm, Em, F, G, Am, Bm7b5
Chords from the Am scale are...................... Am, Bm7b5, C, Dm, Em, F, G
Same notes in both, same chords in both. If the song's harmony (chords) revolve around the C, F & G chord or the major chords in the key of C then your song will be, and sound, major. If the song's harmony (chords) revolve around the Am, Dm & Em or the minor chords in the key of Am then your song will be and sound minor.
You gotta look at the music and see which way it's using the major or minor chords. The key signature tells you the major and
relative minor key. You look at which set of chords are being used to decide if the song is major or minor.
Where is the A (major) on the circle? Answer; at 3:00 O'clock. How many sharps does the A (major) scale have? Answer; three. Question: Why is the bottom of the circle so crowded? Answer; there are 7 notes in each scale so they have to wrap around 6:00 O'clock.
Lot of good stuff hidden within the circle --- if you know what to look for. Here is a 20 minute video on the Circle - lot of good information - stick with him as long as you can. http://www.youtube.com/watch?v=FmqZH7nEqvs