The root note in a major scale has a major chord
The 2nd note has a minor chord
the 3rd note is a minor chord
the 4th note is a major chord
the 5th note is a major chord
the 6th note is a minor chord
and the 7th note is a diminished chord
so in C: C, Dm, Em, F, G, Am, B dim
you can apply this to any key and if you want a sad song for instance you might focus on the minor chords in the key
but the best way to learn anything in music is to use your ear!
