Connecting Arduino Mega to HTC one SMartphone via usb

I am new around here and i recently bought a Arduino Mega Adk board. I want to connect the arduino to my smartphone via usb cable and control it with my phone, for example to make a led blink. I couldn't find how to do this, so i will appreciate every advise you tell me.

I couldn't find how to do this

You couldn't find how to do what? To write a program that runs on your phone to talk to the serial port? To write a sketch on the Arduino that listens to the serial port and does something with the input?