I don't recall what Minecraft's figures are formatted in.... usually there are some free 3d party apps like Milkshape3d, XNALara for exporting models from games (proprietary models are never to be sold, only used for creating art and even then some companies are skiddish about it.). Valve has a 3d video program for creating movies.

http://www.sourcefilmmaker.com/

Tutorial https://www.youtube.com/watch?v=xG02JftoiLU

The program is free, but if you download Valve models it's a big download. You do need a Valve account (and an install of Steam) to use it.

Minecraft skins are pretty easy to make and fully supported by Minecraft. Here's a tutorial on skins at Instructables

http://www.instructables.com/id/Your-Easy-Guide-to-Minecraft-Skins/?ALLSTEPS

On a side note, WoW Blender is fantastic. Just discovered this older video on exporting/importing a world to Blender. Mind you it's an older video but should still work.

https://www.youtube.com/watch?v=8Bs3wCfmV_U

It might be easier to sculpt a figure using either Blender or Wings3d (both free). Both can do some UV mapping. On rigging, you could rig it in poser (using the poser bone system). You'd have to read a tutorial or several... Hope this helps a bit.

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