1) Open up a new workbook.
2) Go to Format | Styles...Click modify
3) Select the Alignment tab & pick the options you want as the defaults for your workbook. click OK etc...
4) Enter some text to see the effect.
So, the issue now is how to get this default setup to apply to all your new workbooks. As styles are one of the things that can be stored in templates, one answer would be to save the workbook as a template (*.xlt) and base future workbooks on that template. See the help topics on:
'Settings Microsoft Excel can save in a template'
'Customize the defaults for a workbook or worksheet by using a template'