Excel - How to number rows

I can't figure out how to use the auto number routine in Excel 2010 so I made this one up.

Sub NumberCol()
  Columns("A:A").Select
  Selection.Insert Shift:=xlToRight, CopyOrigin:=xlFormatFromLeftOrAbove
  Range("A1").Select
  Selection = 1
  C = ActiveCell.Address
  D = Cells.Find("*", SearchOrder:=xlByRows, SearchDirection:=xlPrevious).Row
  E = "A1:A" + Trim(Str(D))
  Selection.AutoFill Destination:=Range(E), Type:=xlFillSeries
End Sub