What's notepad?
Anyhow, I guess you mean Notebook Layout. Selecting text and using the Font
menu works fine. To change it permanently, while in Notebook Layout, select
Format | Font, set a font and click Default. Say yes to the message about
changing the defaults. You'll notice that it offers to change the Note
Level styles, not the Normal default style.
(On quit, you may get a message about saving changes to Normal. Say yes.
Alternatively, holding down Shift and clicking File | Save All should
pre-empt that message by saving Normal immediately.)