Due to complexity of being able to mod Commandos, you will require some knowledge of C++, or related languages.
If you want to learn C++ from ground up, a few basic tutorials, head over to http://www.cprogramming.com.
Unfortunatly, another complexity is the need of a good compiler . Commandos was compiled under Visual C++. I am sure you can fetch Visual C++ 6 for no more than $400, looking on Ebay is excellent as well.
If you want some more tutorials on C++, or you would like to learn OpenGL and DirectX! (oohH!!!), head over to http://www.gamedev.net.
If you're like me, and you want to learn even further, head over to NeHe.
OpenGL: http://nehe.gamedev.net
Hopefully this helped.
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