Craft of Electronics Tutorial: Turning an LED On http://craftofelectronics.org/

led.ino

// Turning things on and off.
void setup () {
  // First, tell the Arduino we want to make one
  // of our pins an OUTPUT pin.
  pinMode(13, OUTPUT);
}
  
void loop () {
  // Tell the Arduino to turn on one of our pins.
  digitalWrite(13, HIGH);
}