Unity includes JsonUtility for simple serializable data.

[System.Serializable]
public class SaveData
{
    public int coins;
    public int level;
    public string playerName;
}

Serialize:

string json =
    JsonUtility.ToJson(
        data,
        true
    );

Deserialize:

SaveData data =
    JsonUtility.FromJson<SaveData>(
        json
    );

JsonUtility works best with simple serializable classes and fields.