If you are starting out developing mods for World of Warcraft and need a place to start. The AddonTemplate mod is very helpful. As a mod it does nothing in the game. It does however provide with a starting point to begin the process of learning how mods and Addons work.
If you learn best by learning from others this is a great tool. You can look at how this World of Warcraft mod is constructed and model your own addons after it.
Also don't forget to take advantage of Blizzard's Official Interface AddOn Kit. It is of great help to those starting out writing Mods.