Suppose you are creating a game and you need a 3D object for your game.
For example, you need a 3D shooter man in your game. You can design any 3D object using Blender.
Here in this tutorial, you can design a 3D submarine and add animation also to it. For this you need Blender 3D software. You can download Blender 3D from here. You can also use 3DS MAX.
The pdf file contains step by step process to design a submarine.