1. Think up a random word or phrase, a genre of game's, a thing or activity you like, you choose what you do here.
tip: make sure its one thing, we will expand on the idea later

2. Think of a way you can present this in a creative, new , and unique way.
Mash it with something if you can't find a new way to present this.

3.Expand on you idea, what is the story behind your application, gameplay, everything that is necessary for production.
You could draw a quick sheet.
E.g.
Name: Lasers
What its about: You use your laser to shoot the zombies and keep them away from your base, the longer you survive the more zombies come out you,
Controls: Use mouse pointer to aim and shoot