How to create xml or json data and return it, as a web service, in C#?

I am creating a C# ASP.NET Web Service application. I need to access data from a SQL database and return this data as XML or JSON. When you run the HelloWorld method (see the code below), it returns the data in XML. But I need to make my own XML tags from the SQL data. How is this done?

In an addition, how can I make the web service use JSON, not XML?

As an example, say I'm returning a table set of 2 rows and 2 columns from SQL and I format it in XML:

<returnSet>
  <row1>
    <FirstName>
      David
    </FirstName>
    <LastName>
      Faustino
    </LastName>
  </row1>
  <row2>
    <FirstName>
      Henry
    </FirstName>
    <LastName>
      Irons
    </LastName>
  </row2>
</returnSet>

Thank you very much for any assistance.

UPDATE

I modiifed the code below for the inclusion of returning JSON. The problem is when I debug the application and click on the method, it still shows XML:

<?xml version="1.0" encoding="utf-8" ?>
<string xmlns="http://tempuri.org/">"Hello World"</string>

My expectations have been write the code below, then goto Debug => Start Debugging. Click HelloWorld and, voila, it returns data in the form of JSON. I see elsewhere that some are calling it using jQuery or plain JavaScript. Is that the only way to test that it's returning JSON data?

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.Services;
using System.Web.Script.Services;
using System.Web.Script.Serialization;

namespace WebApplication2
{
    /// <summary>
    /// Summary description for WebService1
    /// </summary>
    [WebService(Namespace = "http://tempuri.org/")]
    [WebServiceBinding(ConformsTo = WsiProfiles.BasicProfile1_1)]
    [System.ComponentModel.ToolboxItem(false)]
    // To allow this Web Service to be called from script, using ASP.NET AJAX, uncomment the following line. 
    [System.Web.Script.Services.ScriptService]
    public class WebService1 : System.Web.Services.WebService
    {

        [WebMethod]
        [ScriptMethod(ResponseFormat = ResponseFormat.Json)]
        public string HelloWorld()
        {

            JavaScriptSerializer js = new JavaScriptSerializer();

            return js.Serialize("Hello World");
        }
    }
}