Remember how I showed you the SaveFileDialog? Well you can use the OpenFileDialog in the exact same way with a couple minor tweaks:

vb.net Code:
  1. Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
  2.         Using ofd As New OpenFileDialog With {.AddExtension = True, _
  3.                                               .DefaultExt = "txt", _
  4.                                               .Filter = "Text File (*.txt)|*.txt|All Files (*.*)|*.*"}
  5.             If ofd.ShowDialog = Windows.Forms.DialogResult.OK Then
  6.                 Me.TextBox1.Lines = IO.File.ReadAllLines(ofd.FileName)
  7.             End If
  8.         End Using
  9.     End Sub