When you open the Visual Basic editor, you could find something called
ThisWorkBook. In this, you have to write the following code:

Private Sub Workbook_open()
Application.ScreenUpdating = False

Worksheets("Sheet1").Visible = False
Worksheets("Sheet2").Visible = False.

Application.ScreenUpdating = True
End Sub

So, when you open the workbook, the workbook_open event gets
triggered, which automatically executes the lines in that event
procedure. So, thereby you could make ur sheets invisible.

ScreenUpdating is set to false, becoz you dont want any screen
flickering when the processor is processing the task. You may not
find big difference for this process, but when you want to process a
large value and enter each value into the cells, then if you dont
provide that statement, you cud see the screen getting flickered,
which is not a good thing to see. also, lot of memory is wasted for
it, which indirectly slows down the process (ie) processing time is
larger.