Kate,

How exciting. I'm pretty new with VBA but I just created a macro that
does what you explain. You'll want to create a "Worksheet_Change"
Private Sub. As an overview, Worksheet_change procedures run whenever
there is a change in the worksheet in general. However, you can tell
it to look at a specific range. See below. This is not tested but
pulled from the macro I just wrote (which works).

Private Sub Worksheet_Change(ByVal Target As Excel.Range)
Dim VRange As Range, Cell As Range
Dim ValidateCode As Variant
Set VRange = Range("your_cell_here")
For Each Cell In Target
If Union(Cell, VRange).Address = VRange.Address Then
ValidateCode = EntryIsValid(Cell)
If ValidateCode = True Then
**code here**
Exit Sub
End If
End If
Next Cell
End Sub

Private Function EntryIsValid(Cell) As Variant
If Cell = "B" Then
**code here**
Else
exit sub
End If
End Function

Hope this makes some sense. Good luck!
Christine