When you come up with your idea, it usually isn't complete. You will need to add a goal and a challenge to complete it. Build your game incrementally by adding first the goal and then the challenge.
Everything starts with you coming up with an idea. But this is only the start!
Next step is to verify your game has a goal in and that the goal is clear and understandable. If you don't have a clear goal, you should come up with one that is a clear, simple and easily to understand. Test it with others if possible by simple telling them about the idea and the goal in the game. You can also provide a drawing.
Third part is to involve some sort of challenge. The challenge provides difficulty to the game and makes it meaningful to play. A without any challenge is not meaningful. Very much related to this is the Learning Curve.
Lets say our idea would be Tetris. In the game you need to match the dropping blocks to the bottom of the screen by rotating them to remove the rows before the screen is filled.
For Tetris the goal is to keep the screen clear and not to hit the top of the screen. You do this by creating full rows which are then removed.
For Tetris the challenge is that you need to turn the dropping pieces around for the best possible position before they hit the bottom. As you advance in the game, the speed of the blocks increases which yet adds to the challenge.