So Animefan already told you about the client-jar, and that you can go into .minecraft/versions to directly open the client jar.
The minecraft wiki is also useful, as it breaks down the json formats and tells you what is available and what each element does. https://minecraft.gamepedia.com/Model
Also make good use of google, and the search bar right on these forums. Try asking your question in different ways, class and method names are typically helpful when searching for stuff to help narrow it down.
I think the main thing is to not get frustrated, nor to give up. And of course, once you really feel that you've looked and tried your best, you can always come and post on here, just make sure your explanation is complete, and you provide code and logs (github is your best friend for this), and people here are usually good at spotting things we miss.