By extension I assume you mean file format.
There is no simple answer. Any coder can make their own format to do anything they like, and game coders seem to really relish doing as such. Worse is they are not always great coders so they may reinvent the wheel when the rest of the world has moved onto spacecraft. If nothing else look no further than the idea of custom text encodings when the rest of the world settled on a handful of known formats for many many many years now.
Pull apart games on the same system
Pull apart games from the same developer
Pull apart games in the same series
If there is a SDK for the system in question you might get something from it -- most of the GBA and DS devs making games used a sound format that Nintendo pushed that are known as Sappy and SDAT respectively. Newer systems do better than older ones for this sort of thing but older SDKs can still be valuable.
See if anybody else has pulled apart games from the same developer, system and series. Though game devs seem to have a bit of penchant
Float around game specific websites -- if you wanted to hack pokemon then a few hours floating around https://www.pokecommunity.com/
will likely get you far more than anything a more general Nintendo game hacking forum would get you.
You can also float around general websites like http://wiki.xentax.com/index.php/Game_File_Format_Central
and the wiki here.
In the end though you are still going to have to learn how the file formats are made, the hardware they are for (I like nice abstract things but hardware does not and as you have to run it on hardware you have to play to it). People approach it in different ways depending upon their backgrounds, the systems they are working on (learning SQL is not going to help you hack NES games, helps tremendously for a lot of online games and world of warcraft and all sorts of things) and how tied to a given system they want to be.