Andriod control on/off via usb cable at Arduino motor ouput

I would require to connect a andriod phone to a ardunio via usb so that on the andriod phone screen i can open a andriod application and have a option on and off. When the on button is pressed it should turn on the motor which is connected to the arduino output. when off it should turn off the arduino output motor. Thus equipement use is a andriod phone, Ardunio MEGA 2560( one of the digital pin output connected to trigger the motor, usb cable(connection for the andriod phone to the andriod mega) and DC motor(used at the arduino digital output pin.)