Super Mario Bros. X Forums

I mean, where should I start? I want to learn coding with lua, and I know it is much more simple than it looks, but I am not sure on what should I begin with.

Find some tutorials for Lua. Lua.org or the lua-users wiki would be a good start.
Then, once you understand Lua a bit better, you should look at the SMBX-specific stuff here. (Don't go here first or I can 100% guarantee that you'll be confused.)

Learn general Lua. Practice some coding and then move to LunaLua. I've been watching this guy's videos as of late and most of his content here was really helpful

Skipped through the first one and it seems pretty messy and it looks like he skips over a lot of important stuff. He doesn't do a good job at explaining why stuff works the way it does, leaving people clueless as to why things work the way they do.
I like the tutorials Sambo linked, because they're clearly structured and elaborate. Definitely check those out.