This is the basic example that comes as one of the examples when you download the arduino software, I use this to give the board a quick test.
In the picture below you can see this running on an Arduino MEGA 2560 clone board, the LED has the letter L beside it.
Here is the code that comes with the example
/*
Blink
Turns on an LED on for one second, then off for one second, repeatedly.
This example code is in the public domain.
*/
// Pin 13 has an LED connected on most Arduino boards.
// give it a name:
int led = 13;
// 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 (HIGH is the voltage level)
delay(1000); // wait for a second
digitalWrite(led, LOW); // turn the LED off by making the voltage LOW
delay(1000); // wait for a second
}
Now compile and upload the example and the LED on the board will blink on for one second and then off for a second