It all depends on what you'd like to do. For 3D modelling, your best bet would be to try out the free programs like Anim8or, Wings3D and Blender (though Blender has a pretty steep learning curve compared to the other two). The bulk of the CG art on Renderosity uses Poser, which you can buy from CuriousLabs.com -- I think they have a demo to try, as well.