I guess you are trying to Call REST based service which will return Json Data.
Firstly check for valid REST Service Url. If your REST Service url is correct then try calling service by below code.
string apiUrl = "YourFullServiceUrl";
Uri address = new Uri(apiUrl);
System.Net.HttpWebRequest request = System.Net.WebRequest.Create(address) as System.Net.HttpWebRequest;
request.Method = "GET";
request.ContentType = "text/json";
using (System.Net.HttpWebResponse response = request.GetResponse() as System.Net.HttpWebResponse)
{
System.IO.StreamReader reader = new System.IO.StreamReader(response.GetResponseStream());
string strOutput = reader.ReadToEnd();
}