You can use LitJson, it is easy to use and easy to install.
Go to this link and download and import the .unitypackage into Unity:
https://github.com/Mervill/UnityLitJson
Demo code (c#):
using LitJson;
public class myClass : MonoBehaviour
{
private void Start()
{
string jsonString = "{ "distanceThisMonth": { "jeff": 56, "lizibod": "206.02" }, "distanceThisWeek": { "jeff": 13, "lizibod": "37.36" }, "stepsThisMonth": { "jeff": 94838434, "lizibod": "316812" }, "stepsThisWeek": { "jeff": 235642, "lizibod": "57331" } }";
JsonData json = JsonMapper.ToObject(a JSON string);
Debug.Log(json["distanceThisMonth"]["jeff"].ToString()); //returns 56
Debug.Log(json["stepsThisMonth"][1].ToString()); //returns 316812
}
}