string jsonData = @"{
'FirstName':'Pietie',
'LastName':'Massimo'
}";
var details = JObject.Parse(jsonData);
var firstName = details["FirstName"]
or
dynamic details = JObject.Parse(jsonData);
var firstName = details.FirstName;
//You want to convert it to an object first and then
//access normally making sure to cast it.
JObject obj = JObject.Parse(json);
string name = (string) obj["Name"];