cgpeers have some good courses.
It usually follows basic simple rules:
Square = Stable, Strong, reliable
Circle = Friendly, happy, good
Triangle (spiky) = Evil, danger
Those are like the bread and butter of character design.
I tried to design a mario OC clone, and made pic related.
as you can see, his major principal body shapes (silloethe) are basically squares, and he has a lot of squares, like his hand and the yellow patch where he holds his tool.
There's also his face which is rounded which makes him friendly as secondary major shape.
So, you need to look up into the psychology of shapes, I'm sure there are some youtube tutorial about it.