Game source

If it’s an online game that run with JS, you can modify it’s source code with the developer tool of your browser (the inspect element).

Otherwise if it’s a game that you have downloaded, you’ll only have a binary file. In this case you can’t access the source code because you will only access the compiled file (if you have already use a compiled language like C/C++ you know what I mean). In this case, you can use Reverse Engineering to try to understand the code behind the binary data and potentially find a vulnerability. Reverse Engineering is an exciting topic, it’s also very wide and you’ll need a lot of knowledge, but I’m sure you can find some tutorial online and some good books in the libraries.

Wow it quite complicated but thanks I’m only an newbie

I’ve retrieved the first tutorial I’ve followed : here

depends what game if its a steam game, then there is guaranteed to be checks on bootup or account checks, but r4v explained it very well also word of warning any modified call of duty games are patched after each boot, you need to find a way to reinject it quickly everytime you play (this is where my interest lies at)