help needed in VSTS unit testing...

hi,

i'm very new to the world of .net..i have given a task to unit test the methods. some methods i did successfully..here is the method where i'm stuck..can u help me in unit testing the method.

Method goes some thing like the bellow.

public FreeTimeFrame[] GetAllAvailableAppointments(DateTime startDate, DateTime endDate)
        {
            FreeTimeFrame [] availableAppointments;

            GlossyWebService.GlossyWebService service = new GlossyWebService.GlossyWebService();
            service.Url = ServiceDetails.Url;
            AppointmentServiceRequestMessage appointmentServiceRequestMessage = new AppointmentServiceRequestMessage();

            appointmentServiceRequestMessage.StartDate = startDate ;
            appointmentServiceRequestMessage.EndDate = endDate ;

            AppointmentServiceResponseMessage appointmentServiceResponseMessage = service.GetAvailableAppointments(appointmentServiceRequestMessage);

            //TODO refactor to proper error handeling of service errors.
            if (appointmentServiceResponseMessage.ServiceError.ToString() == "no service error")
            {
                availableAppointments = appointmentServiceResponseMessage.FreeTimeFrames;
            }
            else
            {
                throw ExceptionFactory.RaiseSystemException(new CustomException("Service error: ", appointmentServiceResponseMessage.ServiceError));

            }
            return availableAppointments;    
        }