Getting started with writing addons

#REDIRECT Create a WoW AddOn in 15 Minutes
The idea of picking up any programming language can be daunting, and when you have to tie your program into someone else's APIs it can be nearly overwhelming. Creating your own addon will be a difficult process for novice coders, but the end satisfaction is worth it.
See also: Writing a HelloWorld AddOn, Blizzard DebugTools.
== First Steps ==
Figure out what you want your addon to do. Don't try and get into the details too much, just decide what you want the overall goal to be. I usually start by asking myself these questions:
:*What's my end goal?
:*What information will I need from WoW for my addon to work?
:*Will I need to make custom frames?
:*Will I need to interact with Blizzard's frames?
:*Do I know the functions needed to do everything I want to do?
=== Get Your Information: The WoW API ===
Getting information from WoW is critical to making your addon function. The World of Warcraft API has a list of functions that will give you that information. These are your building blocks, and if you don't have a block you need (or thought you needed) then you'll have to find another way to get information for your addon. Don't be discouraged, as with any programming language there are many different ways to accomplish your goal, so keep looking until you find a way.
You should also get the source code of Blizzard's default interface. This allows you to view how existing functionality is implemented in the interface, as well as find examples of how some API functions are used.
== Editing Tools ==
Before you can write any sort of code, you'll want to have a tool that lets you edit you addon files. All addon files are plain text files, meaning even Notepad will do. You can also get Lua and XML specific text editors, which will highlight syntax for you, in turn making your coding a lot easier.
To review the list of tools and get one you'd like, head to the Lua editors page.
== File Types ==
There are three primary types of files that you'll need to worry about with addon editing:
; TOC File : This file is required for any addon. This is the file that supplies WoW with information about your addon and which files are required for your addon to work.
; Lua Files : This contains all the programming for your addon. 
; XML Files : This holds the layout of frames you have created for your addon.
===TOC File===
This is the file that instructs WoW in some basics about your addon and what other files you want loaded. Here's a short example of one:
 ## Interface: 
 ## Title: My AddOn
 ## Notes: This AddOn does nothing but display a frame with a button
 ## Author: My Name
 myAddOn.xml
You can read more about what you need or can put in a TOC over at The TOC Format
=== Lua Files ===
Lua files contain functional pieces of code. You may choose to only have one of these or break up your code into multiple files for a specific purpose. Here's a short example of one:
function MyAddon_OnLoad(self)
	SlashCmdList["MyAddon"] = MyAddon_SlashCommand;
	SLASH_MYADDON1= "/myaddon";
	self:RegisterEvent("VARIABLES_LOADED")
end
=== XML Files ===
XML files are used to specify the visual style of your frames. More importantly though a frame allows you to easily pass events to your Lua files. Check out XML User Interface for details. Here's a short example of an XML file:
 <Ui>
 	<Script file="MyAddon.lua" />
 	<Frame name="MyAddon">
 		<Scripts>
 			<OnLoad function="MyAddon_OnLoad" />
 		</Scripts>
 	</Frame>
 </Ui>
== Start Coding! ==
The HOWTOs here has a ton of great examples to help you learn. Don't be shy to dig through someone else's addon for help, just be sure not steal code and give credit where credit is due.
You may especially start by creating a very basic "Hello World" AddOn as described in the AddOn programming tutorial Introduction.
== Slash Commands ==
A slash command is one way for the user to interact with your addon. These will be the easiest way for a beginner to let the user supply command and change options.
The HOWTO: Create a Slash Command page covers this pretty well.
== A Basic UI Frame ==
The XML User Interface page covers a lot of a great basics.
== SavedVariables ==
The Saving variables between game sessions article covers the key points. For folks new to Addons but not other programming languages, just bear in mind that:
* The SavedVariables is only read from on UI loading, not real time, and only saved to on UI unloading. This will generally be from a user logging in or logging out.
* Basically, WoW makes a SavedVariables file for you named after your addon. You only get to specify in your TOC *which* variables get saved into that file.
== Localization ==
It's a good idea to plan from as early as possible to have your addon be localizable, even if you yourself only speak one language. Review this great article for some things to think about: HOWTO: Localize an AddOn.