Isometric is very different from 3d modeling and texturing. You need to experiment first. A lot.

Get yourself Blender or Milkshape. Though I recommend getting commercial 3d apps earlier on (3dsMax, Maya, Cinema4d), if you want to make this your profession.
Joining GPL game development groups however is a good way to learn more.
So for now, get a modeling app and learn tutorials. Doesn't matter if it's not for nexuiz yet. Just learn as much as you can.
Also take note that making game resources is very different from making movie/art resources. You need to be conscious of the number of polygons you are using since high-poly models would make it unusable ingame (the cause of most lag problems are from unoptimized 3d resources).
Anyway, good luck!
