For creating new models, you need GMax with Renegade Tools (RenX) for modeling, PaintShop, Photoshop or similar for the textures, a simple text editor (for coding), a lot of time, coffee and creativity

A lot of modeling tutorials and solutions for several problems are spread over the WWW. Google (or any other search engine) and the board search are a very good help for modding CnC.

GMax can be found here:
Turbo Squid -- gmax
RenX is here:
RenX and W3D Viewer - FileFront.com
Greets
G.S.