Start with editing models, that way you'll learn the components off models and how to solve most of the troubles.
Scratch modelling requires a lot of practice (unless you want to make shitty models with parkinson animations), the best is that you start with some practice by editing existing models. Afterwards, get yourself a good modelling program (3dsmax, maya, or gmax) and start going easy: try merging both techniks, scratch and editing. I, for example, started by editing existing models changing a bit the geometry and then reunwrapping, next I started animating using maxbones & IK(which was a pain to convert because there wasn't a tool to do so), later I merged edits with scratch made parts and finally I made full scratch models(but no longer for wc3).