I'd like to point out that writing for a book is different than writing for a film and that is also different than writing for a videogame.
Things that work in one may not work in the other. For example, extended inner dialog works well in a book, but it becomes boring quickly in a movie, while it works in a game if it's not essential and it's done during an activity that is not too demanding (so the player can split his attention to both the task and dialogue).
As usual, all I can advise is to start small. Write a two page story GEARED TOWARDS GAMING (more doing, less contemplating and pondering), and then try to take it to completion.
I'm not much for big stories in games, but what I've seen that works is focusing on what is to be done and why. Don't stray too far into hypothetical future or ancestors or whatever, as it will be normally glossed over and forgotten, unless you're doing a big franchise, in which case you wouldn't be in here anyway
Having a backstory is nice, but you'll usually rely on it for better characterization. Resident Evil 5 does this well, serving "files" (text backstory) on each character, while the main game only shows the present and some hints of the past. This keeps things moving.