I would like a message box to pop up when text is entered into the shipping address as a reminder to check correct address...

I am using
Private Sub Worksheet_Calculate()
If [B12] = "" Then
    MsgBox "CONFIRM SHIPPING ADDRESS"
End If
End Sub
But this only works if it is blank... also, it comes up every time any cell is changed
I know the simple problem is right in the "IF [b12] = "" but i dont know how to fix it...
Very simple, but i am total illiterate in VBA nearly... also i only need it to do it once when something is typed in that cell...