Can anybody help me understand them better ? With a link to a guide or anything ?

Well,

First you will need to understand the language you are in. Then probably understand the maths of the problem (if involved).

Look for keypoints. If I try to understand quake3 code, I will try to search for the keyword "Rocket". And from there, try to find where it is allocated. And so on to finally understand how the rocket launcher works, and weapons, then entities, and so on.

If a code is badly commented, and is not using recognizable design patterns, you will need patience :D
You might need someone to explain it to you. (The creator of the code)

Well see I wanted to make my own code/game that is very small that I can practice on and experiment .

Oh and yeah I understand C++ a bit, and I was thinking about learning C# I think it is, because it has double the make time I'm pretty sure I should look into it more.

"Double the make time?" What do you mean by that? I can't think of anything for which that makes sense -- in fact, the closest thing I can think of (compile time) would make that statement trivially untrue: C# typically compiles faster than C++ as project complexity increases.

Care to explain yourself? I'm afraid you may have been mislead.

Start at the beginning. Learn about programming - what it is, how it works, how to solve simple problems, then how to use abstractions in the process of solving larger problems. Other people's production code - that is, code that wasn't specifically written for the purpose of learning from - is a horrible teacher, and you'll gain little from it if you don't already know what you're doing.

If you're new to programming it might be a good idea to choose a language that is a little less complex than c++, though this really depends on how tenacious you are in the learning process (as in: it's possible to start with c++ but be prepared to do a lot of reading/researching/cursing).
Secondly, it's important that you understand that teaching yourself the skills needed to develop a game might take a while, so don't expect to be making sonic the hedgehog clones in 5 months.
Thirdly, you need to word your questions better if you want to get efficiënt help from whatever community you pose questions to. This means teaching yourself english (if this isn't your first language), getting to know the proper programming terms (such as function, parameter, pointer, etc...) and giving enough context to your question.
For instance: you asked how to learn "codes", without mentioning what language you want to learn, what you aim to achieve and what you already know.

I know this post doesn't directly address what you asked for, but it'll help you just the same.