OK, you've opened the VBE. In the left pane look for:
VBA Project(YourSpreadsheetName)
and drill down until you see its 'This Workbook'. Right-click on this
and choose 'View Code'.
In the new (blank?) area that appears, there should be 2 dropdown
lists at the top, in the left dropdown choose 'Workbook', and if the
right dropdown doesn't already show 'Open', choose it there. The
flashing cursor should now be sitting somewhere inbetween
'Private Sub Workbook_Open()'
and
'End Sub'

Paste the following code there:
msgbox "hello"


Close the VB Editor, save close the workbook, and then open  it again. QED.

In VBE duoble click on Thisworkbook (in project explorer window)
and copy this code

Private Sub Workbook_Open()
MsgBox "Helooo"
End Sub