It depends on what you want to do. If you just want to do things like add attachments/particle emmiters, you only need
Yobguls' File Converter and
information about editing MDLs. Some examples of what that can do are
these Infernals.
If you want to edit verticies, faces, and whatnot, follow
this tutorial.
Creating new models from scratch is your last and probably hardest method. You'll need a modeling program such as
Milkshape(30 day trial, then
only 25$ to register) or
Gmax(free). There are various exporters that change the file formats of this programs into the file formats used by Warcraft III. The Milkshape exporter is
here and their will be an exporter released for Gmax soon, or maybe it's already realeased. Anyway, there are lots of helpful tutorials in the
tutorials section.
