The way I 'learned' lua was by looking at other people's codes (on CivFantaics), and then trying to understand what they did. An alternative would that to be to download mods (but NOT those gigantic total conversion mods; start with the "simple" and smaller mods) and look at how they achieved to code their UA/UU/UB/whateverelse.
That being said, I already knew several things about the basics of coding (though Lua is a tad bit different in some syntax, the idea is the same), and I bet that helped me a lot trying to understand Lua. So any experience in stuff such as java(script), php, python, GameMaker Language, or even something as simple as Scratch would already help out a lot! If you don't, you could try to search for "The basics of coding" in Google, which I bet would contain some great tutorials/explanation videos!
Now, I'm nowhere near being a Lua master (heck, I'm still a novice), but I've found out that simply trying to code stuff has learned me a lot. Don't be afraid to open up a thread if you don't know where to start coding your UA (or whatever else), don't be afraid to open up a thread to ask a specific question regarding Lua, and don't be afraid to open up another thread if you've gotten somewhere with your code, but don't know how to continue! (Or create your own 'inquiries-thread', which many people (including me) have done so that they won't spam the forums
)
Anyhow, Good Luck!