pinMode()

Configures the specified pin to behave either as an input or an output. pinMode(pin, mode) pin the number of the pin whose mode you wish to set mode INPUT, INPUT_PULLUP, OUTPUT none
void setup() {
  pinMode(20, OUTPUT);      // sets the digital pin as output
}

void loop() {
  digitalWrite(20, HIGH);       // sets the LED on
  delay(1000);                  // waits for a second
  digitalWrite(20, LOW);        // sets the LED off
  delay(1000);                  // waits for a second
}