How to use payment gateway in asp.net c#

Call This Function

 string StrStatus = fillOnlineAmount();

                if (StrStatus == "SUCCESS")
                {
                    st = 1;
                }
                else if (StrStatus == "FAILED")
                {
                    st = 0;
                }




 private String fillOnlineAmount()
    {
        string url = "http://www.payeservices.co.in/api_users/balance?username=xxxxx&pwd=xxxxx";
        HttpWebRequest httpreq = (HttpWebRequest)WebRequest.Create(url);
        HttpWebResponse httpres = (HttpWebResponse)httpreq.GetResponse();
        StreamReader sr = new StreamReader(httpres.GetResponseStream());
        string results = sr.ReadToEnd();
        txt_mainBalance.Text = results;
        sr.Close();
        return results;
    }