1. There a bunch of different types of software you can use to convert Minecraft saves to .obj models. I use mcobj , which is a small command line program, but best for dealing with the large save files we want. Download mcobj version 0.14 .
2. To render the model once we've got it, we'll need a 3D modelling package. If you have thousands of dollars/pounds, you can use Autodesk Maya or 3D Studio Max. I use Blender though, which is free, robust, and has a good set of tutorials. Set that downloading too.