View and hide columns in excel using vba

Question

I have a worksheet with values in columns B:G. In the same sheet in cell A1 I have made a drop down list using data validation with values like A, B and C.

What I requir

Answer 1

Go to view --> macros.
Hit the dropdown and do "record new macro". Right click on a column header and do hide column.
Then do unhide column. Do Macros->stop recording. Macros-->View macros Click edit.

you get the following code:

Columns("C:C").Select
Selection.EntireColumn.Hidden = True
Selection.EntireColumn.Hidden = False

Now you know how to hide and show columns. First you select the column then your set Hidden = true or false.

Google: excel macro when cell value changes

Click the first link: http://support.microsoft.com/kb/213612

Take the code from that link and read the comments:

Private Sub Worksheet_Change(ByVal Target As Range)
    Dim KeyCells As Range

    ' The variable KeyCells contains the cells that will
    ' cause an alert when they are changed.
    Set KeyCells = Range("A1:C10")

    If Not Application.Intersect(KeyCells, Range(Target.Address)) _
           Is Nothing Then

        ' Display a message when one of the designated cells has been 
        ' changed.
        ' Place your code here.
        MsgBox "Cell " & Target.Address & " has changed."

    End If
End Sub

Make sure you read the link very closely. And follow the instructions. I find I sometimes rush and miss important details

Let me know if this is enough or you need more help.