Pick a simple goal or idea for your AddOn. It's often a good idea to just make an AddOn that only opens a window or prints a message to get started, especially when everything is new for you. After that works the way it should, and you have learned everything necessary to make an AddOn work at all, you can move on to adding one piece at a time toward your goals. Writting software in general is often a never ending process because you can always think of something new to add or improve.
The World of Warcraft API has a list of functions that you can interact with and to talk with WoW using Lua. These are your building blocks for manipulating WoW windows and chat boxes and so forth. Looking at these for the first time can be very daunting. But don't be discouraged. After getting more familiar with Lua the WoW API gets much easier, and the documentation gets much easier to read. Keep in mind there are many different ways to accomplish your goals, so when going though the API keep looking until you find a way that best suits what you need.
Our old friend Quest Helper isn’t updated for the client, but there’s !Questie and MonkeyQuest to help you track available quests and their objectives. The biggest challenge to leveling up in the beta is searching for where the quest objectives are, so if you’re looking to hit the level 30 cap fast — or just the level 5 cap in today’s stress test — you’ll need one of these.
If you're looking for simple, just get actionator. Set it to undercut everything you post for 1c and alt click from your bags. It will place the item on the ah window, do a scan for that item and autoset the price for how much you told to undercut (you can change it accordingly if you feel like it). It will remember if you do stacks for which items and replicate it everytime.