I recommend you start with this one, because that's how I started.
http://xenforo.com/community/threads/creating-an-addon.5416/
After that, use the tutorial addon you created and expand it to your idea.
Then I proceeded on watching Kier's videos. I watched that video at least 10 times just to get it in my head.
http://xenforo.com/community/threads/xenforo-ajax-tutorial.8091/
Once you know how code them, it'll feel so rewarding because you can now build in-house addons and make your ideas come true. So far I created 3 in-house addons and addons that extends/overrides existing addons in the Resource area. I know you can do it, too!