Exception error in the first webdriver compilation

Hi, I am trying Selenium WebDriver for the first time.
Program:
import org.openqa.selenium.WebDriver;
import org.openqa.selenium.firefox.FirefoxDriver;

public class firstjavaclass1 {

public static void main(String[] args) {
WebDriver d = new FirefoxDriver();
d.get ("http://bbc.com");
}
}

Error:
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 https://github.com/mozilla/geckodriver. The latest version can be downloaded from https://github.com/mozilla/geckodriver/releases
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:244)
at org.openqa.selenium.firefox.FirefoxDriver.<init>(FirefoxDriver.java:219)
at org.openqa.selenium.firefox.FirefoxDriver.<init>(FirefoxDriver.java:214)
at org.openqa.selenium.firefox.FirefoxDriver.<init>(FirefoxDriver.java:210)
at org.openqa.selenium.firefox.FirefoxDriver.<init>(FirefoxDriver.java:123)
at firstjavaclass1.main(firstjavaclass1.java:7)