This can be maddeningly complex, depending on which platform and version of Excel you use. Note: biggest challenge with this tip is figuring out the right location for the template file. As long as you name the template correctly, and put it in the correct location, Excel will use your custom template to create all new workbooks.
If so, you can save yourself time and trouble by setting a default template for Excel to use each time you create a new workbook. Do you find yourself creating new workbooks in Excel, then making the same changes to every one? Maybe you like to change font size, zoom percent, or the default row height?