Try this simple solution:
Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
Dim filedata = My.Computer.FileSystem.ReadAllText(CurDir() & "\keys.txt")
Dim sp() = filedata.Split(vbNewLine)
Dim IndexOfStart = Array.FindIndex(Of String)(sp, AddressOf CheckStr)
Dim linecontainigStart = sp(IndexOfStart)
End Sub
Function CheckStr(ByVal line) As Boolean
Dim tmp = InStr(line, "Start")
If tmp > 0 Then
Return True
End If
Return False
End Function