void setup() {
  Serial.begin(9600);
  pinMode(13, OUTPUT); 
  Serial.println("hello"); 
}

char rx_byte = 0;

void loop() {
  if (Serial.available() > 0) { 
    rx_byte = Serial.read();
  
    switch (rx_byte) {
      case '1':
        digitalWrite(13, HIGH);
        Serial.println("LED is ON");
      break;
      
      case '2':
        digitalWrite(13, LOW);
        Serial.println("LED is OFF");
      break;
      
      case '3':
        Serial.println("------- MENU -------");
        Serial.println("1. LED on.");
        Serial.println("2. LED off.");
        Serial.println("3. Menu.");
        Serial.println("--------------------");
      break;
      
      default:
        Serial.println("Erorr");
      break;
    } // end: switch (rx_byte)
  } // end: if (Serial.available() > 0)
}