I am a player of GTA games. I want to create my own car for GTA: Vice City. How do I create a mod that adds a new car to the game?
To make a totally new car, you'll first have to model it in 3D in a modelling program. Blender is a reasonable freely-available modelling tool.
Once you've got the model, you need to export it into the
.dff file format used by Vice City. This plugin can export from Blender to
.dff; you can find plenty of other
.dff exporters for other modelling tools as well if you search around.
The G-IMG tool can be used to edit GTA's archive files and install your custom
.dff (and associated
.txd files exported for the car are the basics needed to get the car in game; the rest allows you to customize and tweak the car.
This guide covers the basic steps in more detail.