First off you have to acquire a 3D modelling suite and become reasonably good with it. I wouldn't recommend trying to jump right into modelling without knowing first knowing quite a lot about 3D modelling and how it works.
You have two choices, either go down the free route and get
Blender, or
if you have a lot of money at your disposal, get
3DSMax. I personally prefer this program, although a lot of others much prefer Blender.
After you know your way around the program enough, search around here for modelling tutorials. There is an excellent one by Timu Simisu, which I can't seem to find at the moment, but it's around here somewhere.
When you learn how to accurately make a model that meets the standards of TA or any other graphic update projects around here (I don't think there are any others, actually), then you can create and contribute models you've made.
Also, this almost definately isn't Tech-related, so post in the appropriate section next time.