Multiplayer On One Device

A good start is to watch some of the tutorials on http://unity3d.com/learn and understand how to begin your games. It's hard to help someone who simply has a concept and no specific question.

Have you begun your game at all? If not and you haven't seen the tutorials I highly recommend you complete the tutorials (Watch and follow along) to gain a basic understanding of the Unity API.

It also may be beneficial to learn C# or Javascript if you don't already have a fundamental understanding of either one (Or at least some prior programming/scripting experience).