just start to learn how to use the BTT. I want to customize some features in on touch bar. However, most of them are requiring code. What language should I learn for this? and what should I learn and notice for advanced using BTT?
Depends very much on what you want to achieve. BTT can trigger scripts of almost any programming language, but most macOS automation is currently done using Apple Script.
I would recommend to first look at some of the user made presets in the preset section.
okay do u know any place that I could learn Apple Script?
Honestly Apple Script is not really worth learning in detail (it's not a great language). For most things it's enough to google some snippets and put them together 
There is the official Apple documentation, but it's really overkill https://developer.apple.com/library/archive/documentation/AppleScript/Conceptual/AppleScriptLangGuide/introduction/ASLR_intro.html
thanks