Select 1st item in a List box in a User form?

I have added a List box to a User Form and the User Form is activated when the sheet is selected. The 1st item in the List box is not automatically selected, however. What code do I need to add to select and highlight the 1st item? This is the code I am using to display the user form.

Private Sub Worksheet_Activate()
    UserForm1.Show vbModeless
End Sub

Thanks in advance