DaFool has posted a tutorial in the Generals Tutorials Warehouse which explains the majority of the basics on Generals Modding. I asked him to write one up based on Lin Kuei Ominae's Tiberian Sun guide, and he has and posted it today.
The tutorial goes over four different sets of information.

I - File Types
II - Setting up the Generals / Zero Hour directory properly
III - Important .INI files
IV - Important tools, what they do, and where to get them.

Go here to check out the tutorial.