usando o codigo

import org.openqa.selenium.By;
import org.openqa.selenium.WebDriver;
import org.openqa.selenium.WebElement;
import org.openqa.selenium.firefox.FirefoxDriver;

public class TesteAutomatizado {
  public static void main(String[] args) {
      WebDriver driver = new FirefoxDriver();
      driver.get("http://www.google.com.br/");
      WebElement campoDeTexto = driver.findElement(By.name("q"));
      campoDeTexto.sendKeys("Caelum");
      campoDeTexto.submit();
}
}

deu o erro
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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:754)
    at org.openqa.selenium.remote.service.DriverService.findExecutable(DriverService.java:124)
    at org.openqa.selenium.firefox.GeckoDriverService.access$100(GeckoDriverService.java:40)
    at org.openqa.selenium.firefox.GeckoDriverService$Builder.findDefaultExecutable(GeckoDriverService.java:114)
    at org.openqa.selenium.remote.service.DriverService$Builder.build(DriverService.java:329)
    at org.openqa.selenium.firefox.FirefoxDriver.toExecutor(FirefoxDriver.java:150)
    at org.openqa.selenium.firefox.FirefoxDriver.<init>(FirefoxDriver.java:120)
    at org.openqa.selenium.firefox.FirefoxDriver.<init>(FirefoxDriver.java:98)
    at TesteAutomatizado.main(TesteAutomatizado.java:8)