So, your ball should have an X and Y coordinate. You need to introduce a Z coordinate which reflects the height above the ground. When rendering, render the shadow at (X,Y) and the ball at (X,Y-Z).
Bonus points for introducing juice by deforming the ball when it leaves the ground and when it lands
EDIT: just realised you probably want some help with the physics too
You could do this quite simply by introducing the following concepts:
velocity: (X,Y,Z)
gravity: floating point value
friction: floating point value in 0.0-1.0
At every step, you'd do:
velocity.Z += gravity
position = position + velocity
Then check for collision with the ground (Z <= 0) and respond appropriately.
NOTE: don't forget about adjusting the formulae for a fixed timestep...