Это довольно просто, брат:
Private Sub FillRichTextBoxFromFile(ByVal path As String)
Try
If IO.File.Exists(path) Then
Using sr As New IO.StreamReader(path)
Dim s As String = ""
Dim i As Integer = 1
While Not sr.EndOfStream
s += CStr(i) + ". " + sr.ReadLine + vbNewLine
i += 1
End While
RichTextBox1.Text = s
End Using
Else
MsgBox("Oooops, File not found !!!")
End If
Catch ex As Exception
MsgBox(ex.Message)
End Try
End Sub