To modify fonts, move the font folder to your main World of Warcraft directory (i.e C:/Program Files/World of Warcraft). Copy your preffered font into this fonts folder, and name it accordingly:

FRIZQT__.ttf (the main UI font - NPC names, player names, spell names, item names, buffs, quest log text, combat text, button text)
ARIALN.ttf (chat font, info text, small text)
skurri.ttf (default unit frame combat text)
MORPHEUS.ttf (mail text, quest log header text)
FRIENDS.ttf (friends list text)

~http://vranx.com/font.htm