To start your mod, and modeling you will need this 3 things.
Softimage Mod Tool (440 MB)
Free for Non-commercial Game Creation and Modding. Only available by filling out the trial download form on the Autodesk website.
Autodesk® Softimage® Mod Tool software (formerly XSI Mod Tool) is a free* 3D modeling and animation package for creating non-commercial games and "modding" (modifying games). Autodesk® Softimage® Mod Tool 7.5 is based on the same professional toolset found in Autodesk® Softimage® 7.5 software, used by game developers to create 3D characters, levels, props, and animation.
Forge Tools 3 (31 MB)
Free from Sins of a Solar Empire (SoaSE), Forge Tools 3 includes the latest files for map making, modding. Updated for Sins 1.15/Entrenchment 1.01.
Graphics
A graphics control tool like Photoshop or Gimp (free).
For Photoshop you will need DDS Nvidia Plugin.
This is what I got so far and have read the pdfs on Forgetools3/Documentation. So far so good.