This example shows how to blink an external LED using the GPIO0 pin of the ESP-01.

void setup() {
  pinMode(0, OUTPUT);
}

void loop() {
  digitalWrite(0, HIGH);  
  delay(1000);                          
  digitalWrite(0, LOW);   
  delay(1000);                          
}

The LED is connected to GPIO0 through a resistor (≈100Ω) and then to ground.