Your json is a property called "data" which is an array of objects, and in your case there is only one item in the array. If you wanted to get the id of the first item in the data array you would do this
string sp = Convert.ToString(obj.data[0].id);
However if you expect more items to be in data then you'd need to loop to find the one you wanted, or to process them all, whichever meets your requirements.