Here is the information from Excel Help:

Insert a new sheet that is based on a custom template
1.If needed, create the worksheet template that you want to base a new worksheet on.
How to create a worksheet template

1.Select the worksheet that you want to use as a template.
2.Click Microsoft Office Button , and then click Save As.
3.In the File name box, type the name of the worksheet template.
To create a custom worksheet template, type the file name that you want to use.
To create the default worksheet template, type sheet.
Note Custom templates are automatically saved in the Templates folder. The default worksheet template, sheet.xltx or sheet.xltm, should be saved in the XLStart folder, which is usually C:\Program Files\Microsoft Office\Office12\XLStart.
4.On a computer that is running Windows Vista:
In the list, click Excel Template or Excel Macro-Enabled Template.

On a computer that is running Microsoft Windows XP:
In the Save as type box, click Excel Template or Excel Macro-Enabled Template.
5.Click Save.
2.Right-click the sheet tab of a worksheet, and then click Insert.
3.Double-click the template for the type of sheet that you want.