Hey Hansraj, to verify an error message on a webpage, you can use getText() and Assert method in Webdriver:
import org.openqa.selenium.By;
import org.openqa.selenium.firefox.FirefoxDriver;
import org.testng.Assert;
import org.testng.annotations.Test;
public class TestNaukri {
    
@Test
public void TestError()
{
  
  FirefoxDriver driver=new FirefoxDriver();       
  driver.manage().window().maximize();
  driver.get("http://www.naukri.com/");
  driver.findElement(By.id("p0submit")).click();
        
  // This will capture error message
  String actual_msg=driver.findElement(By.id("emailId_err")).getText();
    
  // Store message in variable
  String expect="plz enter valid email";
  // Verify error message
  Assert.assertEquals(actual_msg, expect);
    }
}
For further understanding, you can refer to the Selenium Course.