|I didn't understand how a car (or wagon) starts its motion.
A wagon starts its motion when you pull it or push it. If its wheels weren't touching the ground, they would simply move along with the wagon and would not turn. However, they are touching the ground and the ground exerts a backward frictional force on them to keep them from sliding on the ground. This backward frictional force causes the wheels to begin turning.
A car starts its motion when the engine of the car exerts a torque on its wheels. These wheels begin to rotate. However, the wheels are again touching the ground and the ground exerts a frictional force on the wheels to keep them from skidding. This frictional force not only opposes the wheels' angular acceleration, it also causes the wheels and the car to which those wheels are attached to accelerate horizontally.