You'll have to go into the css file of your skin and make the changes. If you are using one of the default skins, look at the skin.css file located under root\portals\default\skins\DNN-Blue\skin.css. For help with the different css classes, look at this site: http://www.skinningtoolkit.com

Also there is a seperate css file for the module containers too.

To get a global font you can also set a global declaration like

body, td, p, div, span {font-face: arial;}

just make sure it doesn't get overriden by another class