Just take note that it depends on the game you want to mod.
Nintendo games are well known for being easy to mod, since each resource is in a separate file.
Other games, such as those created with CryEngine, tend to package all the resources into one file, or even worse, into the program RPX itself, which means that you will have to decompress and disassemble the program to find the resources.
The more common games (Mario Kart 8, Smash Bros. etc.) have resources on modding.