I am just starting to learn the LUA programming language this year, and I have a few questions.
What is the most efficient way of learning?
Please let me know which one is the best.
Another question.
How long does it take to learn LUA on average?
I’d say the most efficient way to learn to code is through help forums and looking at the API itself, either through the lua page or the devwiki. Alternatively, you can go to the luau site. All of these sources have good examples, are pretty easy to understand and don’t spend 20 minutes rambling, like most youtubers do.
Going to give my two cents on how I learned just for variety in responses under this post.
I originally started by watching Peaspod’s tutorials on YouTube forever ago. He covers the basics in his tutorials although I’m not sure if that series holds as much value to this day since things have changed. After leaping over your first hill and understanding the basics, try posting on the DevForum as you’ve mentioned not only asking for answers to your problems, but understand why that solution worked.
Imo, I wouldn’t just jump into the wiki with minimal knowledge of Luau. Learn the terminology before trying to understand the material. Parameters, functions, events, variables, callbacks, arrays, dictionaries, libraries, connections, etc. just to name some of the more common ones.
TL;DR:
When I started a few years ago I started by watching AlvinBlox tutorials, reading DevForum posts, and taking note of the API References and tutorials on the Developer Hub.
Good luck learning Luau! 
This may be a little bit off topic but I learnt from him as well. An old gem but his tutorials will be a bit dated today as they were filmed back in 2015 or so.
If you want to gain a working understanding of Luau, you can just throw yourself at empty scripts and references until you eventually get the outcome you want, improving your muscle memory and knowledge over time.
However; as Luau evolves to be more complicated, an actual fundamental understanding of good practices becomes increasingly important.
You can familiarize yourself with Lua syntax using Programming in Lua (Online Edition). This is the official learning guide for Lua, and understanding it will help you in your journey for Luau literacy. For everything you learn in Luau, I recommend learning its actual documentation and practices (either from Lua.org or another trusted site). Do not accept that a concept just works - understand why it works, and you will progress faster.
You can write Lua 5.4 with this online interpreter, for additional and easy practice. This interpreter is to vanilla Lua what the Command Bar is to Studio.
What you’ll come to realize about learning from random sources, is that most people only know half of what they’re actually talking about. This is why it is important to gather a list of resources that you trust and use them to build your foundation as opposed to some untrusted resource.
Some trustable resources for Lua documentation:
Lua reference manual
Lua-users directory
Some trustable resources for Luau documentation:
Roblox API Reference Manual
You should be cross-referencing everything you learn somewhere else while you build your foundation.
Making simple Projects, like a collectable coin System.
To be honest, becoming a newbie takes about a day or so. Again, I’m just giving my opinion how long it took me.
Then becoming a decent scripter takes about let’s say 1 month or 2. (Took me long, didn’t it?)
Becoming an advanced scripter takes at most 5 months or so.
Now, how do you learn scripting?
I learnt scripting from AlvinBlox, TheDevKing, and watching other tutorials. Then after I knew about devforum, got a lot of inspiration for scripting, and this is where I am right now.
Hopefully you learnt something new! Good luck!