How to secure webservice ?

Question

i have developed webservice using webmethod

[WebMethod]
public string HelloWorld()
{
      return "Hello World";
}
public UserDetails User;
[WebMethod]
[SoapHeader("User", Required = true)]
public string SayHello(string userName)
{
      if (User != null)
      {
         if (User.IsValid())
             return string.Format("Hello...{0} ☺ ", userName);
         else
             return "Error in authentication";
      }
      else
      {
            return "Error in authentication";
      }
}

All working Fine

 

but issue is when i run from desktop app its data (request and response) shown in web debugging software Like Fiddler.

 

even SOAP header is Shown.

(see attached screenshot)

 

How can i secure this Type of web Service (web method)