Create an empty workbook, and modify the Normal style to use a bold font.
Save the workbook as an Excel template named Book.xltx in your Excel startup folder (by default %appdata%\Microsoft\Excel\XLSTART)
Also create a version of this template with only one sheet, and save it as an Excel template named Sheet.xltx in the same folder.
Excel will use Book.xltx when you create a new workbook by pressing Ctrl+N or clicking New on the Quick Access Toolbar, and it will use Sheet.xltx when you insert a new worksheet in a workbook.
PS Please turn off your CapsLock key.