Installing an Addon:
Place your addon inside your WoW Folder>Interface>Addons>
Getting Addons:
All of the following sites are very good, Curse-gaming is generally known for being the best
www.curse-gaming.com/mod.php
www.ui-worldofwar.net
www.wowinterface.com/downloads/
http://www.ctmod.net/
Recommended Addons:
<A href="http://www.ctmod.net/downloads.ct">CT Mods</A>
<A href="http://www.curse-gaming.com/en/wow/addons-1850-1-mobinfo2-includes-mobhealth.html">MobInfo-2</A> - Shows Mob's Health (legal)
Editing an Addon:
Any files that end in .lua,.toc,or .xml will be openable with Notepad / Wordpad
.Lua Files contain the soul of the addon, and most of the coding
.xml Files can be bindings, or the physical parts you can see
.toc Files are 'Table of Content' files, and are the parts that show when you click the Addons button at character selection
I would reccomend not messing with this unless you know what your doing.
Problems:
1) Whenever there's a patch, the addons will be considered out of date, and you will need to click the "Addons" button and check the "Load Out of date Addons" checkbox