I am assuming that you want to create test case where you open the different webdriver related to different browser.
You need to start by creating a testNG class, here you will create test cases for different browsers. Then you will create a xml file where you will create the different class and test cases.
Here is a sample testNG class that I created that works for Chrome and Morzilla Firefox.
package testNG;
import org.openqa.selenium.WebDriver;
import org.openqa.selenium.chrome.ChromeDriver;
import org.openqa.selenium.firefox.FirefoxDriver;
import org.testng.annotations.Parameters;
import org.testng.annotations.Test;
public class MultipleTest
{
    WebDriver driver;
    String url = "https://www.google.com";
    @Parameters("browserType")
    @Test
    public void invokebrowser(String browserType) throws InterruptedException
    {
        browserType = browserType.trim();
        if (browserType.equalsIgnoreCase("chrome"))
        {
            System.setProperty("webdriver.chrome.driver","C:\\Users\\Downloads\\chromedriver.exe");
            driver = new ChromeDriver();
        }
        else if(browserType.equalsIgnoreCase("firefox"))
        {
            System.setProperty("webdriver.gecko.driver", "C:\\Users\\Downloads\\geckodriver.exe");
            driver = new FirefoxDriver();
        }
        else
        {
            System.out.println("Invalid Browser");
        }
        driver.manage().window().maximize();
        driver.get(url);
        Thread.sleep(2000);
        driver.close();
        
    }
}
Here is the XML file for the testNG class 
<!DOCTYPE suite SYSTEM "http://testng.org/testng-1.0.dtd" >
<suite name="ParellelTesting" parallel = 'tests'>
    <test name = "Chrome Testing">
        <parameter name = 'browserType' value = 'chrome'/>
        
            <classes>
                <class name = 'testNG.MultipleTest'/>
            </classes>
    </test>
    <test name = "Firefox Testing">
    
        <parameter name = 'browserType' value = 'firefox'/>
            
            <classes>
            
                <class name = 'testNG.MultipleTest'/>
                
            </classes>
    
    </test>
</suite>
Hope this will help.