How to create a C# Encrypt-Decrypt Program using Visual Studio 2013 Express and Windows Forms?

I've been wanting to create a encrypt decrypt program for a while now and basically I would like to hook up 3 text boxes together with one being text to encrypt. the encrypted code, and the hash used to encrypt the text. so if the program is restarted you can input the hash and the encrypted text to decrypt. Here's what I've got far:

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;
using System.Security.Cryptography;

namespace WindowsFormsApplication1
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void aboutToolStripMenuItem_Click(object sender, EventArgs e)
        {
            MessageBox.Show("This Program was created by Ashar Ahmed. You may not redistibute this work without explicit premission from its creator. \r\n\r(C) 2015 All Rights Reserved to Ashar Ahmed Industries LLC.");
        }

        private void richTextBox1_TextChanged(object sender, EventArgs e)
        {

        }

        private void richTextBox2_TextChanged(object sender, EventArgs e)
        {

        }

        private void richTextBox3_TextChanged(object sender, EventArgs e)
        {

        }

    }

Please help me