On Text Changed in Textbox, Result Must Be Recalculated.

Question

I am newbee. I am trying to make a unit converter. I have 2 textboxes and 2 listboxes.First textbox is for entry, second one is for result. Listboxes are for units.

I don't want any submit button. When I entering first textbox, the second textbox must show result on every changing in first textbox. OR listbox changings must change the result in the textbox2.
My very primitive codes are here. What are my mistakes?

CODE:
using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;

namespace Conversion
{
  public partial class Form1 : Form
  {
  public Form1()
  {
  InitializeComponent();
  {
 
  listBox1.SelectionMode = SelectionMode.One;
  listBox1.SetSelected(0, true);
  listBox2.SelectionMode = SelectionMode.One;
  listBox2.SetSelected(0, true);

  Double DoubleSayi = Double.Parse(textBox1.Text);
  Double DoubleKatsayi1 = 1;
  Double DoubleKatsayi2 = 1;

  Double DoubleSonuc = DoubleSayi*(DoubleKatsayi1 / DoubleKatsayi2);
  textBox2.Text = DoubleSonuc.ToString();

  switch (listBox1.SelectedItem.ToString())
  {
  case "kilometer": DoubleKatsayi1 = 1000; break;
  case "hectometer": DoubleKatsayi1 = 100; break;
  case "decameter": DoubleKatsayi1 = 10; break;
  case "meter": DoubleKatsayi1 = 1; break;
  case "decimeter": DoubleKatsayi1 = 0.1; break;
  case "centimeter": DoubleKatsayi1 = 0.01; break;
  case "milimeter": DoubleKatsayi1 = 0.001; break;
  case "micrometer": DoubleKatsayi1 = 0.000001; break;
  }
 
  switch (listBox2.SelectedItem.ToString())
  {
  case "kilometer": DoubleKatsayi2 = 1000; break;
  case "hectometer": DoubleKatsayi2 = 100; break;
  case "decameter": DoubleKatsayi2 = 10; break;
  case "meter": DoubleKatsayi2 = 1; break;
  case "decimeter": DoubleKatsayi2 = 0.1; break;
  case "centimeter": DoubleKatsayi2 = 0.01; break;
  case "milimeter": DoubleKatsayi2 = 0.001; break;
  case "micrometer": DoubleKatsayi2 = 0.000001; break;
  }


  }
  }

  private void Form1_Load(object sender, EventArgs e)
  {

  }
}
}
END OF CODE