Newton's first law says that an object at rest will stay at rest and an object in motion will stay in motion unless acted on by an unbalanced force. When the person on kicks it is the unbalanced force that acts on the skateboard that is at rest, when the skate board is at a constant speed and it hits a curb the curb is the unbalanced force that makes the skateboard in motion stop moving.
Newton's third law says that any action of force will have a corresponding reaction. When the baseball is first pitched the force is pushing it toward the hitter, when the hitter hits the ball the reaction is that the ball goes some where out into the field.
Newton's second law says that the force is equal to the mass times the acceleration. If the mass of the basketball is heavier it will have less acceleration so the force will be lower. If the basket ball has a lighter mass it will have more acceleration so the force could be greater.