When you are in the Visual Basic editor, click View, Project Explorer. This will expose the individual sheets in the current Workbook. You will also see one named ThisWorkbook. If you double click it, it will open up into the code window for it. If you drop the General combobox in the upper left corner, you will see a choice for WorkBook. When you select WorkBook then the Declarations area on the right side will show all of the WorkBook events available to you. Click on open and then put your code in the sub declaration it will create for you. Here is an example and Good Luck!
Private Sub Workbook_Open()