Check out blender. It's a fairly powerful, open source 3d modeling tool (also game engine, motion tracker, compositor, sculpter, and more). I used to use it quite a lot back in highschool.

It also has a fairly high learning curve at first, but there's a huge community built around it[2]. Andrew price has some very high quality tutorials[3]

1. https://www.blender.org 2. http://blenderartists.org 3. https://blenderguru.com