Sigh, you have no idea how grand a thing you want to do here. You are asking for the source code of one of Activison's largest IP's - which is protected by copyright law. Even if you manage to decompile the executable, doing anything with it would be illegal. I'm pretty sure the EULA of the game even says that you are not allowed to try and decompile and reverse engineer the game. Any kind of mod you manage to produce that way (which I seriously doubt you would be able to do even with years of experience working as a professional software developer) would most likely result in a law suit.
You should drop this idea and focus on something creative instead - like making your own game.