To start the serial system, in setup function put:-
Serial.begin(9600);
Then to send a message to the PC simply use the serial print function:-
Serial.println("GO");
Will sent the string "GO" to the PC.
are carriage return and line feed that tell the PC the string has finished.
And that has sent a message to the PC.
How you receive that message on the PC depends on what language you are programming it in. Most languages can communicate with the serial port or coms port as it is sometimes called.