You would need to draw out a fairly detailed outline of a variety of aspects of the game. Things like target audience and platform as well as story and a flowchart describing how the user would progress through the game. Storyboards describing cut scenes and important parts and designing some levels, especially in 3D can make for eyecatching stuff. It depends on the game you are developing, the more you can put into an idea you want to present the more you can influence whoever looks at your idea.
www. gamedev.net may be a useful resource. Have all your work copyrighted before you send it away.