Already have an awesome idea for a game? Great! Now put it down in writing and try to be as detailed as possible. You may have heard of a design document, which is exactly what it sounds like; a document full of details of your game, whether it's story content, weapons, items, characters, and so on. Here's a good baseline template for your design document:
Name of the game
Overall environment of the game: Do you want it to be comedic, or dark?
Objectives: Things your player will have to do to resolve the issues in the game
Game Progression: How is the game going to play out? Slow and steady, or fast?
Features of the game: We are talking about SDK (Software Development Kit) level here. What do you want to include? Game Center, Xbox Live, Leaderboards, or online multiplayer?
Characters: This is a fun one to work on. Who is the main character? What's his (or her) background story? What does he (or she) look like? What are they wearing? Who is the girl you meet in...