I have a very simple but fun idea in my mind for our first game—a very childish but fun 2D platform game:
This is Jake, an alien who recently passed his flying license. Jake's parents were very proud of him until, for unknown reasons, he crashed into one of the faraway little planet. He lost most of his ship's batteries and needs to get new ones. You will help him get the batteries so that he can fly back home. It will be an awesome adventure. We will collect anything valuable to sell and avoid the dangers of the mysterious planet. Let's call the game Jake on the mysterious planet.
I hope you like the idea. It is very simple as we have limited time. We will use most of the assets from http://kenney.nl/. They kindly agreed to contribute to this book.
I want this game to be as simple as possible so that we can complete it in the next few chapters. We will create a lot of scripts, prefabs, and possibly some assets, too.
Jake on the mysterious planet will be a 2D infinite scroller game. Jake will run automatically from left to right through the mysterious world. We will give him the ability to jump from platforms to collect coins. He can use them to buy batteries for his ship.
In most cases, when you have your own idea for a game, you will have a vision in your mind. You will know what sort of feel and gameplay you want to achieve. However, you won't know the tiny parts of the game and how they will work together to create the game you want. This is why planning is so important. Developing the game starts in your head, and then you have to create the documentation for your idea—even if the game is as simple as Jake on the mysterious planet. Don't be scared! You may not have to document the complete idea first, but trust me! It's worthwhile to have as much as you can written down.
There aren't any golden rules for game idea documentation. You should aim to write down as much as you want for two main reasons:
I know what I am speaking is very generic, and I am not able to tell you exactly what to do. Your idea is your idea and only you can lead it to success. What I can do, however, is show you how I approach planning.