Exception in thread "main" java.lang.IllegalStateException

I am studying automation testing tutorial by selenium and was writing my first script in java language and have got that message in "Console" at Eclipse.

Exception in thread "main" java.lang.IllegalStateException: The path to the driver executable must be set by the webdriver.gecko.driver system property; for more information, see http://ift.tt/26SIDUn. The latest version can be downloaded from http://ift.tt/1UjgqPJ
at com.google.common.base.Preconditions.checkState(Preconditions.java:199)
at org.openqa.selenium.remote.service.DriverService.findExecutable(DriverService.java:109)
at org.openqa.selenium.firefox.GeckoDriverService.access$100(GeckoDriverService.java:38)
at org.openqa.selenium.firefox.GeckoDriverService$Builder.findDefaultExecutable(GeckoDriverService.java:91)
at org.openqa.selenium.remote.service.DriverService$Builder.build(DriverService.java:296)
at org.openqa.selenium.firefox.FirefoxDriver.createCommandExecutor(FirefoxDriver.java:245)
at org.openqa.selenium.firefox.FirefoxDriver.<init>(FirefoxDriver.java:220)
at org.openqa.selenium.firefox.FirefoxDriver.<init>(FirefoxDriver.java:215)
at org.openqa.selenium.firefox.FirefoxDriver.<init>(FirefoxDriver.java:211)
at org.openqa.selenium.firefox.FirefoxDriver.<init>(FirefoxDriver.java:124)
at automationFramework.FirstTestCase.main(FirstTestCase.java:12)

My Code :

package automationFramework;
import org.openqa.selenium.WebDriver;
import org.openqa.selenium.firefox.FirefoxDriver;
public class FirstTestCase {

public static void main(String[] args) throws InterruptedException {
    // TODO Auto-generated method stub

    // Create a new instance of the Firefox driver
    WebDriver driver = new FirefoxDriver();

    //Launch the Online Store Website
    driver.get("http://ift.tt/1xgcG6b");

    // Print a Log In message to the screen
    System.out.println("Successfully opened the website www.Store.Demoqa.com");

    //Wait for 5 Sec
    Thread.sleep(5);

    // Close the driver
    driver.quit();
}

}

The Tutorial Link : http://ift.tt/2cy7qdC