When u are inside a car your feet are touching the floor of the car , and if you will exert some force on any side of the car from the inside there is a second force acting ,which is exerted by your feet towards the floor of the car.These two forces are equal in magnitude. So they are balanced and the force exerted is always equal to the force exerted by your feet on the floor of the car,Hence it is a reaction force according to the newton's third law of motion so they are balanced and the resultant force is zero .Thats why the car does'nt move.
I hope u got it!