Well, it all depends on what part of the game you want to edit/modify. If you wish to edit the scripting of the game, such as how much damage is done by certain weapons, their costs, etc, you will need to get a .big file editor. Here's a link.
http://www.fileplanet.com/dl.aspx?/planetcnc/generals/utilities/finalbig.zip
You can then use this program to open .big files. Open the ini.big and extract it to a seperate folder somewhere safe. You can then just use notepad to open the ini files and then edit the information in them. Certain files you will probably be interested in is the weapons.ini, factionunit.ini and factionbuilding.ini where you can edit the weapons range, power,etc. and can also edit the cost of units and buildings,etc. Its all pretty self explanatory, they have little comments put throughout the files that tell you what does what.
As for editing models and putting them in the game, that is a whole different bag of trouble. You should first get used to editing and changing things in the ini files. Hope this helps and if you have anymore questions just ask.
I wanna creat new untis for Generals but I dont know the program, you know any?, and thanks for this file and this information:rolleyes: