How to handle popup in selenium

WebDriver driver =new FirefoxDriver();
driver.manage().window().maximize();
driver.get("https://www.vrlbus.in/vrl2013/register_customer.aspx");
WebElement firstbox=driver.findElement(By.name("FIRSTNAME"));
firstbox.sendKeys("chaitra");
driver.findElement(By.xpath("//input[@type='submit']")).click();
String alertmessage=driver.switchTo().alert().getText();
System.out.print(alertmessage);