You may use the concept template below or also the canvas tool.
Download the concept document from VideoGameDesigner-ConceptDocument.doc
And to avoid the most common mistakes, download the VideoGameDesigner-ConceptDocument-not-like-this-version.doc
Here's the parts of the documents and what should be in them.
You can find the concept template here. Never put more than two pages into it. Why? Because it will help you nail your game!
Along with these guidelines runs the concept sample for the Fighter Pilot game.
Who will play your game?
Which platform and why this platform? Also understand your platform, what people on that platform expect and how they play.