int incomingByte = 0;	// for incoming serial data

void setup()        
{
  Serial.begin(115200);    //set the serial connection to be very fast. 115200 bits per second!
  pinMode(12,OUTPUT);      //pin 12 is an output
}

void loop()
{
  //check to see if there are any bytes waiting in the buffer?
  //You will receive serial data if the arduino is plugged into USB and the Serial Monitor is open

  if (Serial.available() > 0)     //If there is something waiting to be read: READ IT!
  {          
    // read the incoming byte:
    incomingByte = Serial.read();

    // say what you got:
    Serial.print("I received: ");
    Serial.println(char(incomingByte));

    //Check to see if the incoming byte is the same as an 'A'. Check the ascii table to know which character you're looking for.
    if (incomingByte == 65) digitalWrite(12,HIGH);    //If the letter is an 'A', turn ON pin 12.
    else digitalWrite(12,LOW);        //If not, turn OFF pin 12.

    //Note the shorthand way of writing an 'if' statement.  This only works if there is 1 command to happen after the if or the else statements.
    //To view the ascii table: 
  }
}