Make an LED obey your code.

Wire the LED

An LED is polarised. The longer leg is normally the anode (+); the shorter leg and flat edge on the package identify the cathode (−). The resistor limits current and protects both the LED and Arduino pin.

Your first sketch

const int ledPin = 8;

void setup() {
  pinMode(ledPin, OUTPUT);
}

void loop() {
  digitalWrite(ledPin, HIGH);
  delay(1000);

  digitalWrite(ledPin, LOW);
  delay(1000);
}

Read the code like an instruction sheet

const int ledPin = 8; gives pin 8 a useful name. setup() runs once and configures that pin as an output. loop() repeats forever. digitalWrite(…, HIGH) drives the pin high, and LOW drives it low. delay(1000) pauses for 1000 milliseconds.

The UNO R3 also has a built-in LED associated with digital pin 13, but using an external LED here teaches the breadboard and current-limiting resistor at the same time.

Experiments

1. Change both delays from 1000 to 250. Predict the result first.
2. Make the LED stay on for 100 ms and off for 900 ms.
3. Move the wire from pin 8 to pin 7 and change only one line of code.
4. Add a second LED on pin 9 and try alternating them.