The following sketch will turn the relay on and off every 5 seconds:

/* Gardening system
 * Lets water our plants on time
 * Relay test
 */

#define RELAY_OUTPUT 9

int i = 0;

void setup() {
  // put your setup code here, to run once:
  pinMode(RELAY_OUTPUT, OUTPUT); // Set the pin to digital output mode
}

void loop() {
  // put your main code here, to run repeatedly:
  if(i == 0) {
    i = 1;
    digitalWrite(RELAY_OUTPUT,HIGH);
  } else {
    i = 0;
    digitalWrite(RELAY_OUTPUT,LOW);
  }

  delay(5000);
}

Upload this sketch to the Arduino Pro Mini using the FTDI cable with the Arduino IDE and once the sketch is uploaded, use a multimeter to check that the relay connects the circuit every 5 seconds.