When you open an Excel file it automatically opens the Excel program to view it.
If you want a default file to open when you start the Excel program, I think it can be done via Tools/Options/General tab where near the bottom you have an "At startup, open all files in..." option.
(This is using Excel in MS Office 2003, not sure if earlier versions are the same).
Quite simple really - if you just click anywhere on the sheet (but probably best to click in the top left hand cell) that you want to open when you open the workbook then save - the next time you open the workbook it will open on that sheet.