Global.asax.cs

using System;
using System.Linq;
using Telerik.Sitefinity.Services;

namespace SitefinityWebApp
{
    public class Global : System.Web.HttpApplication
    {
        protected void Application_Start(object sender, EventArgs e)
        {
            SystemManager.RegisterServiceStackPlugin(new Services.HelloWorldServicePlugin());
        } 
    }
}

Greeting.cs

using System;
using System.Linq;

namespace Services.Response
{
    public class Greeting
    {
        public string Text { get; set; }
    }
}

HelloWorld.cs

using System;
using System.Linq;
using ServiceStack.ServiceHost;
using Services.Response;

namespace Services.Request
{
    [Route("/hello")]
    public class HelloWorld : IReturn<Greeting>
    {
        public string Name { get; set; }
    }
}

HelloWorldService.cs

using System;
using System.Linq;
using ServiceStack.ServiceInterface;
using Services.Request;
using Services.Response;
using ServiceStack.Text;

namespace Services
{
    public class HelloWorldService : Service
    {
        public Greeting Any(HelloWorld request)
        {
            return new Greeting() {Text = "Hello, {0}".Fmt(request.Name) };
        }
    }
}

HelloWorldServicePlugin.cs

using System;
using System.Linq;
using ServiceStack.WebHost.Endpoints;

namespace Services
{
    public class HelloWorldServicePlugin : IPlugin
    {
        public void Register(IAppHost appHost)
        {
            appHost.RegisterService(typeof(HelloWorldService));
        }
    }
}