A game can be with or without a story.
Writing a story gives a fixed beginning.
Without a story the gamer has to imagine.
But what a game needs is Environmental storytelling, good narration and engaging dialogues. Plus lot of things are involved in making a great game an interesting setting, tone, memorable characters, dramatic conflict, and so on.
With environmental storytelling the players discovers and reconstructs what the story is. There’s goals and mission to accomplish and with a reason why the players can experience a better game.
To write a game script you have to think of these questions.
Describe the environment; what does everything look like and what does it feel like?
What is History of the world?
Create a Flowchart overview of the story. https://www.gamasutra.com/blogs/EmanuelMontero/20091106/85856/Story_Flowchart_Diagrams.php
Or google video game script example
Create character descriptions and bios for all the major characters.
Write down quests and mission.
Write interactions with non-player characters
7.What is the accompanying material? Is there music? Sound effects?
- What are the things that are going to happen? What decisions will be made? What rooms or areas are the decisions tied to?
Ultimately a scene must contain
Location : where is it happening.
Characters : The main characters, one time appearing characters and the player
Player Goal: What should the player achieve in this mission
Action: What does it require him to do to achieve the mission. You can also add a cut scene * (Cut Scenes are short animations or movies that come before or after major plot points in your story)*
Flowchart: What happens if he wins, What happens if he fails … so on.
Music: That will be nice to imagine :D
Get inspired and write whatever make you happy first. We can always edit and evaluate it later.
Link to best video game scripts to inspire http://mashable.com/2013/12/24/video-game-writing/
Hope it helps Somal Sahjwani