Start with a modeling program. I use Blender 3D, it's free. I started with it in June (no experience with any other modeling program), and by the end of the month I had mastered it well enough to make good clothing meshes, just by teaching myself and referring to online tutorials. There are other free modeling programs too, which may have more Poser-specific support in the way of tutorials and such. (I don't know of many other Poser users using Blender).