You can start creating your addon with notepad. There are enhanced versions out there like Programmer's Notepad and Notepad++ but for small addons notepad is enough for the budding author.
Next step is to bookmark the following pages at the least:
www.wowwiki.com
wowprogramming.com
Both those sites will give you access to functions/forums/objects etc available for the addon author.
After that, find a small addon that does something simple and see if you can see how it does what it does. Then see what you can do to make it look or do something different. Or, you may already have an addon that has stopped working, that you can now see if you can fix for the other users out there. Especially if the author is unable to at the present time.
If you get stuck, you can also post on the forums and we can help you on your way.
There are 2 ways to creating an addon ( addon.toc, addon.lua ) or ( addon.toc, addon.xml, addon.lua ). In other words. You can either write a pure lua addon or utilise the xml system to design your addon and use the lua to do the other side of things.
Now that that has been said. This link will help you write your very first simple addon.
http://www.wowwiki.com/Getting_start...writing_addons
You can then use that as a template for your others by building on the code/changing the code etc as you need.