how to find out exactly where you kick the ball? How do you determine the angle the ball curves while in mid air?
To decide where to kick, there's quite a few options. For gameplay, the more faked it is, the better it probably will be. You could use raycasts from some backward position towards the ball to determine the 'kick point' to apply impulses at.
That said, imparting an angular impulse will not cause the ball to curve in mid-air. Curving comes about from aerodynamics ("magnus force"), which is out of scope for normal rigid body physics. You can fairly simply simulate the behavior of curving based on the linear/angular velocity, though:
http://en.wikipedia.org/wiki/Magnus_effect
Note that angularVelocity represents the the axis of rotation * angular speed in radians. That's why crossing the linear velocity with angular velocity gets you a vector that points in the right direction.
As far as what parts of the entity can help you work on this, check out the internalLinearVelocity and internalAngularVelocity properties, as well as the associated momentum properties, and applyImpulse/applyLinearImpulse/applyAngularImpulse. The applyImpulse method takes an impulse and location at which the impulse is applied, which automatically computes and applies the angular impulse.
There's a common misunderstanding that you might run into as well: applying an impulse of magnitude X away from the object's center of mass will still change the object's linear momentum by X, instead of less than X. The angular momentum change is different depending on the offset from the center of mass.
