I've just started wanting to check out more of a different angle in WoW as I'm impatiently waiting for WotLK. I have always used Addons for World of Warcraft and I have always wanted to know how I can create them. So I guess, my question is how can I make addons for WoW?

Creating AddOns isn't for the faint at heart as it involves a whole lot of programming but if you have a basic idea of how to write programs you will probably be able to create AddOns fairly well.

One of the awesome things that Blizzard has done for WoW is to let people create AddOns that enhance the gameplay. They have also released the "World of Warcraft Interface AddOn Kit" which has many nice tools in order to help you out in the process.

I'm not going to go in a huge amount of detail about what it takes to make an AddOn but give you more of a getting started guide that will lead you to a basic understanding of how you can create AddOns.

Here's the most important basic structure of every WoW AddOn.

There are 3 basic types of files you will run across when you look at any AddOn:

.lua files, which contain the program code that explains the nuts and bolts of what everything does in your AddOn.

.toc files, which is the Table of Contents for your AddOn. Just like a table of contents in a book it helps identify your AddOn and tells where and what order the AddOn files should be loaded.

.xml files, which describes the look and feel for your AddOn.

If you compare it to a car, the XML file would be the outside cosmetics of the car (paint job, shape, etc.). The LUA files would be the mechanical part of the car (engine, transmission, etc.) and the TOC file would be, for lack of a better comparison right now, the manual for the car. Hopefully that helps you visualize it a little bit better.

That's the very basics of it but a nice little start. Here are a ton more resources that will come in handy if you like programming and become serious about creating AddOns of your own.

LUA Programming Manual
XML Info
WoW AddOn Tutorial - Very good tutorial and more information about WoW AddOns from the EU-Forums.