Как правильно написать http запрос?

        private void post_http(string url, string data)
        {
            HttpWebRequest req = (HttpWebRequest)WebRequest.Create(url);
            req.Method = "POST";
            req.Accept = "text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8";
            req.CookieContainer = cookies;
            req.Referer = "http://vk.com/audio";
            req.Headers.Add("X-Requested-With", "XMLHttpRequest");
            req.UserAgent = "Mozilla/5.0 (Windows NT 6.1; WOW64; rv:17.0) Gecko/20100101 Firefox/17.0";
            req.ContentType = "application/x-www-form-urlencoded; charset=UTF-8";
            using (var requestStream = req.GetRequestStream())
            using (var sw = new StreamWriter(requestStream))
            {
                sw.Write(data);
            }
 
            using (var responseStream = req.GetResponse().GetResponseStream())
            using (var sr = new StreamReader(responseStream))
            {
                var result = sr.ReadToEnd();//ответ со списком
            }
        }

        private void button1_Click(object sender, EventArgs e)
        {
            post_http("http://vk.com/audio", "act=load_audios_silent&al=1&gid=0&id=ID");
        }

    string getAudio(){
        try{
        CookieContainer cook=new CookieContainer();
        Uri url=new Uri("http://vk.com");
        cook.Add(url,new Cookie("remixlang","0"));
        cook.Add(url,new Cookie("remixseenads","2"));
        cook.Add(url,new Cookie("remixdt","0"));
        cook.Add(url,new Cookie("remixsid","**************"));
        cook.Add(url,new Cookie("audio_vol","96"));
        cook.Add(url,new Cookie("remixflash","11.4.31"));
        HttpWebRequest zapros = (HttpWebRequest)HttpWebRequest.Create("http://vk.com/audio");
        zapros.Method="POST";
        zapros.CookieContainer=cook;
        zapros.ContentType="application/x-www-form-urlencoded";
        using (StreamWriter strwr=new StreamWriter(zapros.GetRequestStream()))
               strwr.Write("act=load_audios_silent&al=1&gid=0&id=17738938");
        using(StreamReader postish=new StreamReader(zapros.GetResponse().GetResponseStream()))
               return postish.ReadToEnd();
        }
        catch(Exception e)
        {return e.Message;}
    }

Console.Write(getAudio());