HelpHelp with making program

Public Class Form1
    Private _baseUrl As String = ""
    Private _outputFilename As String = ""
    Private _webRequest As System.Net.WebRequest
    Private _webResponse As System.Net.WebResponse
    Private _startTime As DateTime
    Private _forceStop As Boolean = False


  

    Private Sub cmdStart_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles cmdStart.Click
        Dim startPage As Int32 = 0
        Dim endPage As Int32 = 0
        Dim totalUsers As Int64 = 0 '' sum total of all usersnames found

        If Not Int32.TryParse(txtStartPage.Text, startPage) Then
            MessageBox.Show("Invalid start page number. Must be 0 < x > 1000000")
            txtStartPage.Focus()
            txtStartPage.SelectAll()
            Exit Sub
        End If
        If Not Int32.TryParse(txtStopPage.Text, endPage) Then
            MessageBox.Show("Invalid end page number. Must be 0 < x > 1000000")
            txtStopPage.Focus()
            txtStopPage.SelectAll()
            Exit Sub
        End If
        If endPage <= startPage Then
            MessageBox.Show("Invalid end page..must be greater than start page.")
            txtStopPage.Focus()
            txtStopPage.SelectAll()
            Exit Sub
        End If
        If txtOutputFilename.TextLength = 0 Then
            MessageBox.Show("Invalid output file name.")
            txtOutputFilename.Focus()
            Exit Sub
        End If
        ''User start/stop pages were valid
        _baseUrl = txtBaseUrl.Text
        _outputFilename = txtOutputFilename.Text
        Dim _pageUsers() As String = Nothing
        Dim _tmpSpan As TimeSpan
        _startTime = Date.Now()

        For xx As Int32 = startPage To endPage
            If _forceStop Then
                DoOutput("Force stop. Shutting down.")
                DoOutput("Last page successfully read #:" & (xx - 1).ToString())
                Application.DoEvents()
                _forceStop = False
                Exit Sub
            End If
            lblCurrentPage.Text = "Current Page: " + FormatNumber(xx, 0)
            Try
                _pageUsers = GetUsernamesByPage(xx) '' this function parses the raw page data and pulls out the usernames
            Catch ex As Exception
                '' server was busy? Page unreachable.
                DoOutput("Server busy/unreachable! page # " & xx.ToString() & " (wait 10sec.)")
                Application.DoEvents()
                xx -= 1
                Threading.Thread.Sleep(10000)
                Continue For
            End Try

            System****.File.AppendAllLines(_outputFilename & ".txt", _pageUsers)
            totalUsers += _pageUsers.Length()
            lblTotalUsers.Text = "Total Users: " + FormatNumber(totalUsers, 0)
            lblCurrentUser.Text = "Current User: " & _pageUsers(_pageUsers.Length - 1)
            ProgressBar1.Value = CInt((xx / endPage) * 100)
            _tmpSpan = Date.Now.Subtract(_startTime)
            lblTotalTime.Text = "Total Time: " & Math.Floor(_tmpSpan.TotalMinutes).ToString & " m, " & CInt(_tmpSpan.TotalSeconds Mod 60).ToString() & " s"
            Application.DoEvents() '' hmm
        Next
        _tmpSpan = Date.Now.Subtract(_startTime)
        DoOutput("Done. Time taken: " & Math.Floor(_tmpSpan.TotalMinutes).ToString() & " minutes, " & CInt(_tmpSpan.TotalSeconds Mod 60).ToString() & " seconds.")
        MsgBox("DONE. Time taken: " & Math.Floor(_tmpSpan.TotalMinutes).ToString() & " minutes, " & CInt(_tmpSpan.TotalSeconds Mod 60).ToString() & " seconds.")
    End Sub
    Private Sub cmdStop_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles cmdStop.Click
        _forceStop = True
    End Sub
    Private Function GetUsernamesByPage(ByVal pageNumber As Int32) As String()
        Dim _userNames As New List(Of String)

        _webRequest = System.Net.HttpWebRequest.Create(_baseUrl + pageNumber.ToString())
        _webRequest.Timeout = 10000 '' timeout = 10 seconds
        _webResponse = _webRequest.GetResponse()
        Dim _entirePageText As String = New System****.StreamReader(_webResponse.GetResponseStream, System.Text.Encoding.ASCII).ReadToEnd
        Dim _tableText As String = ""

        '' Find the table that contains all users
        Dim tableStartIndex As Int32 = 0
        Dim tableEndIndex As Int32 = 0
        tableStartIndex = _entirePageText.IndexOf("<tbody>")
        If tableStartIndex = -1 Then
            MessageBox.Show("Unable to find user table inside webpage. Format is out of date! This program needs to be updated!")
            _userNames.Add("ERROR Page #: " & pageNumber.ToString())
            Return _userNames.ToArray()
        End If
        tableEndIndex = _entirePageText.IndexOf("</table>", tableStartIndex)
        _tableText = _entirePageText.Substring(tableStartIndex, tableEndIndex)
        ''Find each row
        Dim _rowStartIndex As Int32 = _tableText.IndexOf("</th><th><a href=")
        Dim _rowEndIndex As Int32 = 0
        Do While _rowStartIndex > -1
            _rowEndIndex = _tableText.IndexOf(" />", _rowStartIndex) + 3
            _userNames.Add(_tableText.Substring(_rowEndIndex, _tableText.IndexOf("</a>", _rowEndIndex) - _rowEndIndex)) '' lol 
            _rowStartIndex = _tableText.IndexOf("</th><th><a href=", _rowEndIndex)
        Loop
        Return _userNames.ToArray()
    End Function

    Private Sub DoOutput(ByVal msg As String)
        txtOutput.AppendText("[" & Date.Now.ToLongTimeString & "] " & msg & Environment.NewLine)
    End Sub
End Class