Unit test for http web request and response in c#

I want to write a unit test for http web request and response method. Please find the below method,

 public string GetEmployeeId()
        {

                var tokenRequest = (HttpWebRequest)WebRequest.Create("http://www.goggle.com");
                tokenRequest.Method = "POST";
                tokenRequest.ContentType = "application/x-www-form-urlencoded";

                var bytes = Encoding.UTF8.GetBytes(GetKeys(credentials));
                tokenRequest.ContentLength = bytes.Length;

                Response response;
                try
                {
                    using (var stream = tokenRequest.GetRequestStream())
                    {
                        stream.Write(bytes, 0, bytes.Length);
                        stream.Flush();

                        using (var webResponse = request.GetResponse())
                        {
                            Stream receiveStream = webResponse.GetResponseStream();
                            StreamReader readStream = new StreamReader(receiveStream, Encoding.UTF8);
                            MemoryStream ms = new MemoryStream(Encoding.Unicode.GetBytes(readStream.ReadToEnd()));
                            DataContractJsonSerializer ser = new DataContractJsonSerializer(typeof(Response));
                            response = ser.ReadObject(ms) as Response;
                            ms.Close();
                            readStream.Close();
                        }
                    }
                }
          return response.Id;

        }

  private string GetKeys(Credentials credentials)
        {
            return String.Format(@"client_id={0}&client_secret={1}&grant_type=client_credentials",
                                credentials.Id, credentials.Secret);
        }

I dont know how to write unit test for web request methods.Can anyone suggest how to write unit test for the above method?