Guys i Need Guidance with high score C#

,so below i have given my codes ,what i am trying to achieve here is to save my score and display it on some other scene ,this score should be saved even when if i close the program and user should be able to share it on social media ,and the other issue is that i don't know how the score keep getting multiplied but somehow it just does get multiplied that just frustrating ,my method of doing it have not worked, could someone guide to the right tutorial or tell me basic idea of it .

 1.    public class callscore : MonoBehaviour {
 2.    
 3.    private GameObject score;
 4.    private score scorescript;
 5.    
 6.    
 7.    // Use this for initialization
 8.    void Start () {
 9.    score = GameObject.FindGameObjectWithTag("mainscore");
 10.    scorescript = score.GetComponent<score>();
 11.    
 12.    }
 13.    
 14.    // Update is called once per frame
 15.    void Update () {
 16.    
 17.    }
 18.    void OnTriggerEnter2D(Collider2D other)
 19.    {
 20.    if (other.gameObject.tag == "bis")
 21.    {
 22.    scorescript.scoreValue = scorescript.scoreValue +1;
 23.    
 24.    Destroy(other.gameObject, 2.0f);
 25.    }
 26.    }
 27.    }