You can use a mod to change in game fonts. Or you can replace the original game fonts with the fonts you want to use. Create the following folder:
<WoW directory>\Fonts\
Create copies of the font file of your choice, place them in the above folder, and rename them to the following names:
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)

Popular Fonts: Expressway Free, Comfortaa, SD Fresh Regular, Tork, Andika Basic, SF New Republic
dafont has many good fonts to choose from.

ChatFontsPlus Killakhan

A simple addon that adds an outline to your chat window font making it visible without a dark background. Also lets you chose a new default chat window font for all your chat windows. Over 40 free fonts included.
Curse, WoWInterface

Fonter Baelual

Allows the use of in game and custom fonts to customize your UI, Allowing you to change any in game fonts to one of your choosing.
Curse

Fontifier Gnack

Fontifier is a font replacement addon for World of Warcraft. Its main goal is to be configurable and easy to use while providing a consistent look by unifying the various font styles and sizes provided by the default user interface.
Curse

gFont Goldpaw

Simple font changer. It features a replacement of (almost) all in-game fonts, as well as the option to active/deactivate them based on group. It divides all in-game fonts into 4 sub-categories, and only affects the font types, not their sizes or styles. This means the addon doesn't break or interfere with other addons that add outlines of shadows to the chat.
Curse

laFont bloodtear

laFont is a simple and lightweight Addon (~0,1kb ingame) which changes the default damage and zone font to a custom one.
WoWInterface

SharedMedia Elkano, Seerah

Inserts media(fonts, bars, borders, etc) into LibSharedMedia's storage. They can then be selected by any addon that supports LibSharedMedia, a lot of them do.
Curse
SharedMediaAdditionalFonts

xDamageFont Xzatly

Change default damage font. Choose your font (www.dafont.com), put it in "xDamageFont\Fonts" folder and rename it "damagefont.TTF".
WoWInterface