First of all, welcome to the club!
The discord tends to have more active users than the forum, so if you want fast answers and activity, that's the best place to go.
As for getting started, you said it yourself. Start with Basic. Just playing around with it and seeing what all the little commands do is a fun learning exercise.
The emulator folder includes a pdf with documentation. The info about Basic is mostly for what's been added to the original Basic V2 language.
Here's a useful link I've found for learning the fundamentals and other things not covered in the CX16 documentation, when it comes to Basic.
https://www.c64-wiki.com/wiki/BASIC
The wiki focuses on the Commadore 64, which this system is based on quit a bit. So it has stuff in common. Just remember that the memory layout is different.
Speaking of "memory" one useful thing to have on hand, is a list of memory addresses. They're referenced as hex codes such as $9F29 etc..
The vera section of the documentation explains a lot, and includes a reference of many memory locations.
The commands "POKE" and "PEEK" let you write to and read from, individual memory addresses. So by peeking and poking to the vera addresses you can do things like: change the graphics settings, set info about your maps, tile sets, sprites, etc..
I also highly recommend using Basload to write your Basic, at least eventually. It's more efficient in many ways, my favorite being that you don't have to type and keep track of all the line numbers. So for large programs, that's worth looking into. It's included with the emulator along with a text editor.
Good luck, and have fun!