We need to know how our game is going to look before we start coding. It is important to have a basic idea and try to imagine whether this idea will work. For your first game, it's highly recommended to start with something small and possibly something that already exists.
Do not start with a complex idea. If it is your first game, you will most likely get stuck and have a hard time finishing it. Pick something you can finish in a matter of days.
Start with a pen and paper or some simple graphics software and try to draw the basic screen from the game. The following figure is an example:
The basic idea is very simple. However, let's try to state even the obvious details. This helps us to make sure we don't forget anything. The main goal of the character is to get as high as possible. The score will be calculated from the height reached. The obvious facts are that the game will be played in portrait mode and that there is gravity that pulls our character down. The not so...