Welcome, good to have you here! It might be tough in the beginning but I guess that goes for all things worth doin'. Enjoy!
Basically you'd just be lifting your middle finger of the fret to go from C to Cm.
Topic: Why does this song sound sad?
Re: Why does this song sound sad?
Well, to me it don't. I guess it's just the association with the lyrics, try singing a happy-go-lucky text over it while playing it slightly faster, completely different feeling.
I hope, for Sepp blatter's sake, that England can turn things around in the second half... Stevie Wonder could have seen that was a goal.