After looking through the wiki a bit and maybe trying some of the tutorials, I'd recommend poking through and trying to modify a relatively small addon that you already use.
There are several challenges in getting started with Addons:
- how files get loaded and how scoping works.
- how lua interacts with the UI xml files
- the actual limitations in the API (this part can be disappointing)
- inspecting the running game to troubleshoot (my command history addon below can make that a little more convenient)
There's a good bit of code to write before your addon can do anything particularly interesting, so be patient.