Firstly, there are two types of hacks you can make:
1.) External (Easier to learn, less powerful and slightly harder to detect by Steam though this isn't an issue with private server version like IW5M/TeknoMW3)
2.) Internal (Harder to learn, possibilities are basically endless as far as imagination goes, easier to detect by Steam, again not a Private server issue)
So pick one. External ones are basically just simple memory editing. (Infinite Ammo, health, Simple ESPs and if you're good enough, aimbots. etc.) while internal ones can make quicker aimbots, better ESPs and many other things much easier.
For external ones, I'd recommend you learn C# (C-Sharp) or VB (Visual Basic). These are based upon the .NET Framework my Microsoft and it's designed for beginners who just got into programming.
Internal ones, C++ - C-Plus-Plus (Which includes C). Rather old but very popular language.
After you've taken the time to learn one, start reading though source codes to improve your knowledge. CoD series are based on the Quake 3 engine so you can download the source and learn from that. MPGH has a very nice Source Code section and you can learn alot from it. If you have any question, feel free to ask.
Avoid begging for source codes. A simple "Could you guys tell me how I'd go about doing xxx" is fine. "Someone give me an aimbot source please" is not.
Basically you need to devote alot of time towards learning if you want to make a decent program.