Well, for a start, do you know how to model stuff in 3d using 3dsmax o softimage xsi? Because my good friend psyko here assumed that you already have a base knowledge of 3d modelling. This is not like re texturing a model, the production pipeline is much more complex than just doing a paintover with photoshop.
I recomend you to go to www.softimage.com
and get mod tool. Then learn the basics of mesh modelling, and then try to make modifications to models.