Pin 8 is motor power, where you should connect the positive lead from the motor battery or the unregulated voltage directly from the main battery.
Pin 16 is IC power, should be connected to regulated positive 5V.
Pins 1, 9 are the enable pins, they should be tied to +5V if you want the motors to operate or to ground (or unconnected) if you want the motors to coast to a stop.
Pins 3, 6 and 11, 14 are the logic input pins, tie them to the microcontroller to drive the motors.
Pins 2, 7 and 10, 15 are the motor output pins, tie them to the motors.
Pins 4, 5, 12, 14 are ground pins.
Now it will work!