Turning a DC motor on and off isn't that much of a problem, is it? But if you want to change direction, you must have a way to switch the polarity that drives your motor. Well, that is what a H-bridge does.
For each motor, you should have 2 pins to control the output. If you pull one pin HIGH and the other one LOW the motor should spin. If you invert this configuration, the motor will spin in the other direction. Pulling both pins to LOW or both to HIGH will stop the motor.
Now you have the ability to turn the motor on in both directions. If you also want to control the speed of the motors, you should connect it to PWM ports.
If your motor doesn't spin, you probably have a too low input voltage. These boards are equipped with voltage regulators. So you can try using a higher voltage. I used a 9V battery for the 5V motors myself. Worked perfectly.
For controlling your robot it makes sense to extract the commands for the wheel to spin in separate methods. Make a method for spinning the left wheel in clockwise direction. And one for counterclockwise direction. It's so easier to program some logic to your vehicle.