Blender is a create free tool to learn modeling in:
http://www.blender.org/Just so you know as well, modding and modeling are not the same thing. Modeling is making ships and different assets.
Modding is changing the game to customize it as you see fit. Wikipedia has a great explanation of it
http://en.wikipedia.org/wiki/Game_moddingThis usually requires some knowledge of programming to be able to do, and we don't know much about how modding in SC will work, though I know I plan on trying it out