The company Adafruit sold the L293D motor shield kit that is now obsolete, but they still provide its learning documentation, Adafruit Motor Shield, at https://learn.adafruit.com/downloads/pdf/adafruit-motor-shield.pdf and the assembled board can be purchased elsewhere. This test program above came from this reference, please refer to it. The AF_Motor Arduino library (AFMotor.h) must be installed to instruct the Arduino how to talk to the Motor shield. Follow the Adafruit instruction on how to install it into the Arduino IDE. Then, copy, paste, and upload this test program to your Arduino and test your car.