C# string array to sort in abc order

here is my code with the comments. so im having trouble exiting out of the loop. i know it shoud be if(contd = true) or something then it should exit out, but also if they enter Y to keep going, how do i get back into the loop? another thing is how to sort this by abc order. and then how do i do the reverse from Z-A. Thanks for your help, clueless beginner here.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace ConsoleApplication1
{
class Program
{
    static void Main(string[] args)
    {
        string lastName;
        string [] lastName = new string[];    //array list of last names
        int index;
        string contd;


        for (index=0;index++)
        {
            Console.Write("Enter a last name: ");
            lastName[index] = Convert.ToString(Console.ReadLine());
            Console.Write("Keep going? (Y/N): ");   
//prompts user to keep inputting or exit out
                contd = Convert.ToString(Console.ReadLine());
                if (contd = "n"(Console.ReadLine());
            {
                //exit out of last name input
            }
            else contd = "y"(Console.ReadLine());
            {
                //go back into last name input
            }
        }

        Console.WriteLine((index+1) + " last names entered");       
//shows how many last names were entered

        Console.WriteLine();                    //spacing

        //display last names
        Console.WriteLine("Names in Acsending Order"); Console.WriteLine(); 

        for(index=0; index++)                   
//shows the last names in order from A-Z (acsending)
        {
            Console.WriteLine(lastName[index]);
        }

        Console.WriteLine();

        Console.WriteLine("Names in Descending Order"); Console.WriteLine();

        for(index=0; index--)               
//shows back last names in reverse order Z-A (descending)
        {
            Console.WriteLine(lastName[index]); Console.WriteLine();

        }
        Console.ReadLine();







    }
}
}