Hello...........There are some excellent lessons/tutorials available free at
http://psrtutorial.com/Resources/R_ChordSecrets/r_chordsecrets.html

They aren't just for Yamaha PSR users either! I used these lessons and now play full chords and "special" chords regularly. I used to be a "single finger" chord player.............darn I wish I learned this stuff earlier! Good luck. -charley