how to make the details save when adding them to the forum on c#

Question

//B00519709 
//Gemma Browse

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
//using arun.CSharp.Namespaces;

//public class friend

    class Program
    {
    static void Main(string[] args)
    {
    Friend[] friend = new Friend[4]; // using 4 different friends details

    int x;
    string firstname;
    string surname;
    string phonenumber;
    int month;
    int day;

    for (x = 0; x < friend.Length; ++x)
    {
    GetData(out firstname, out surname, out phonenumber, out month, out day); // getting the data from the information needed
    friend[x] = new Friend(firstname, surname, phonenumber, month, day);
    }

    Array.Sort(friend);
    Console.WriteLine("Sorted List:");
    for (x = 0; x < friend.Length; ++x)
    Display(friend[x]);

    Console.ReadLine();
    }

    public static void GetData(out string firstname, out string surname, out string phonenumber, out int month, out int day)
    {
    string inString;
    Console.Write("Please enter your friend's firstname: ");    // this is where the user is asked to enter the friends firstname
    inString = Console.ReadLine();
    firstname = Convert.ToString(inString);
    Console.Write("Please enter your friend's surname: ");      // this is where the user is asked to enter the friends surname
    inString = Console.ReadLine();
    surname = Convert.ToString(inString);
    Console.Write("Please enter your friend's number: ");       // this is where the user is asked to enter the friends number
    inString = Console.ReadLine();
    phonenumber = Convert.ToString(inString);
    Console.Write("Please enter your friend's birth month: ");  // this is where the user is asked to enter the friends birth month
    inString = Console.ReadLine();
    month = Convert.ToInt32(inString);
    Console.Write("Please enter your friend's birth day: ");    // this is were the user is asked to enter the friends birth day
    inString = Console.ReadLine();
    day = Convert.ToInt32(inString);
    }

    public static void Display(Friend fri)
    {
    Console.WriteLine(String.Format("{0},{1},{2},{3},{4}",fri.Firstname, fri.Surname, fri.Phone, fri.Month, fri.Day));
    }
    }

    public class Friend : IComparable
    {

    public string Firstname { get; set; }
    public string Surname { get; set; }
    public string Phone { get; set; }
    public int Month { get; set; }
    public int Day { get; set; }

    public Friend(string friendFirstName, string friendSurName, string friendNumber, int friendMonth, int friendDay)
    {
    Firstname = friendFirstName;
    Surname = friendSurName;
    Phone = friendNumber;
    Month = friendMonth;
    Day = friendDay;
    }     

    public int CompareTo(object obj)
    {
    Friend temp = obj as Friend;
    return this.Firstname.CompareTo(temp.Firstname);
    }
    }