3D models are the first step in a 3D print. There are two types of software for creating 3D Models; the first is called CAD, and the second is 3D modeling. 3D modeling is different from CAD in that it's like sculpting with digital clay. It's ideal for creating organic shapes, like characters and textures.
A popular software for 3D modeling is called Blender . It's not only free, but also open source!