What you want cannot be simply done by using an Android device and a USB cable. You will need a micro controller to process the instruction sent from your mobile device and then control the motor movement.
The android device will communicate to the arduino using say Bluetooth(although there are many other alternatives you can select), the arduino will process your instruction sent over bluetooth and then as per your logic it will drive the motor.
To control L298N a single motor using L298N you will use the input terminals 1&2. These will be connected to your micro controller. The motor terminals will be connected to the Output 1 & 2.
Check this image of a micro controller being connected to a motor driver.
L298N is usually used for motors which have a higher current requirement, thus you will also have to look for alternate power supply arrangements too.
P.S. If your motors don't require a lot of power you may consider using L293D which is simpler to use than L298N.