How do I restrict access to some methods in WCF?

I am a bit lost getting started with a simple WCF service. I have two methods and I want to expose one to the world and the second one I want to limit to certain users. Eventually I want to be able to use a client application to use the restricted method. So far I can access both methods anonymously:

C# Code

namespace serviceSpace
{
    [ServiceContract]
    interface ILocationService
    {
        [OperationContract]
        string GetLocation(string id);

        [OperationContract]
        string GetHiddenLocation(string id);
    }

    [AspNetCompatibilityRequirements(
     RequirementsMode = AspNetCompatibilityRequirementsMode.Allowed)]
    public class LocationService : ILocationService
    {
        [WebGet(UriTemplate = "Location/{id}")]
        public string GetLocation(string id)
        {
            return "O hai, I'm available to everyone.";
        }

        // only use this if authorized somehow
        [WebGet(UriTemplate = "Location/hush/{id}")]
        public string GetHiddenLocation(string id)
        {
            return "O hai, I can only be seen by certain users.";
        }


    }
}

Configuration

<?xml version="1.0"?>
<configuration>
  <system.web>
    <compilation debug="true" targetFramework="4.0" />
  </system.web>
  <system.serviceModel>
    <serviceHostingEnvironment aspNetCompatibilityEnabled="true"/>    
    <standardEndpoints>
      <webHttpEndpoint>
        <standardEndpoint name="" helpEnabled="true" 
          automaticFormatSelectionEnabled="true"/>
      </webHttpEndpoint>
    </standardEndpoints>
  </system.serviceModel>
</configuration>

How do I get started?