Global.cs

using UnityEngine;
using System.Collections;

public class Global : MonoBehaviour
{
    private static Global instance;

    void Awake()
    {
        if (!instance)
        {
            instance = this;
            DontDestroyOnLoad(gameObject);
            InitFirst();
        }
        else
        {
            Destroy(gameObject);
        }
    }

    void Start()
    {
        InitScene();
    }

    void OnLevelWasLoaded()
    {
        InitScene();
    }

    void InitFirst()
    {
        // ...
    }

    void InitScene()
    {
        // ...
    }

    // public static functions here:
}