Well, to be a 'modder' in this series is actually a pretty wide ranging question. 'Modding' is doing anything to modify your ThirdWire install, everything from simply providing new loading screens to building aircraft and cockpits.
Most 'modding' can be done with freeware (textures, ini entries, screens, terrains). The big exception of course is 3d modeling.
Skills? Well, it depends.
At a minimum, just to build the model, you need to be able to envision it in 3 dimensional space. The best way to imagine this is being able to take a piece of wood and carve it (or a piece of clay and mold it) to the shape you want. If you have the skills to do this, you probably have the skills to build and manipulate a 3d model.
There are several ways to get proficient using a 3d program...take a class, read a book, follow a tutorial, or just play with it. You need to know the standardized terms 3d programs use, things like mesh, texture, mapping, vertex, polygon, etc.
The best way to build those skills is start simple...build small, simple things, then work your way up. If you try to build a cockpit with your first attempt, you WILL fail, and fail badly.
Others can dive into the details of things like texturing, ini edits, or FM edits.
FC