int switchPin = 2; // switch connected to pin 2
 int relayPin = 3;  // relay connected to pin 3

void setup() {
  pinMode(switchPin, INPUT);
  pinMode(relayPin, OUTPUT);
}

void loop() {
  if (digitalRead(switchPin) == HIGH) { // switch is pressed
    digitalWrite(relayPin, HIGH);        // turn on relay
  } else {
    digitalWrite(relayPin, LOW);         // turn off relay
  }
}