This sketch makes an LED connected to pin 9 to blink on and off every 1 sec

blink.ino

// give a name to the pin to which the LED is connected:
int led = 9;

// the setup routine runs once when you press reset:
void setup() {
    // initialize the digital pin as an output.
    pinMode(led, OUTPUT);
}

// the loop routine runs over and over again forever:
void loop() {
    digitalWrite(led, HIGH); // turn the LED on
    delay(1000); // wait for a second
    digitalWrite(led, LOW); // turn the LED off
    delay(1000); // wait for a second
}