I am trying to read a response from a server that I receive when I send a POST request. Viewing fiddler, it says it is a JSON response. How do I decode it to a normal string
As mentioned in the comments, Newtonsoft.Json is really a good library and worth using -- very lightweight.
If you really want to only use Microsoft's .NET libraries, also consider System.Web.Script.Serialization.JavaScriptSerializer.
var serializer = new System.Web.Script.Serialization.JavaScriptSerializer();
var jsonObject = serializer.DeserializeObject(sr.ReadToEnd());
Going to assume (you haven't clarified yet) that you need to actually decode the stream, since A) retrieving a remote stream of text is well documented, and B) you can't do anything much with a non-decoded JSON stream.
Your best course of action is to implement System.Web.Helpers.Json:
using System.Web.Helpers.Json
...
var jsonObj = Json.Decode(jsonStream);