CSS is an incredibly powerful tool and I'd suggest you get to know how to use it.
This book is a great way to learn it.
To answer your question, to apply a default font to all text on a page, place the following in your <head>:
You can, of course, substitute whatever font values make the most sense for you. The above style rule will apply the styles to all text in the body, as well as within <td>'s (which for some reason are independent of body).
hth,
bear
P.S.
This site is a good reference for CSS.