Try XSI mod tool - its free and you can learn just about everything you need for startup 3D modelling. From there, if you feel like going up a notch or two, 3DSMax is a popular choice for 3D artists, Maya is another popular one and, of course, there's always my favourite, XSI. Full circle sort of thing. 
If you just want to try your hand at ship modelling for Sins, definitely go with XSI mod tool. Its designed for that sort of thing, and importing ships to the game is also easiest.
