I have the idea, the script is written and it looks like a sci-fi novel.
Now how can I make this world "playable"? Do I just put a few players' characters into the scene where a series of actions are taking place, and guide them through the plot until the end?
Writing a loose storyline beforehand is never a bad plan! That said: player actions can make a rigidly-designed story fall apart in seconds. With that in mind: write your story in a way that minimizes the players ability to break it, but doesn't railroad them or deactualize their characters.
This is not an easy task, but there are a fair number of documented theories regarding how this can be done. Of particular note are some articles on The Alexandrian - particularly the articles titled The Three Clue Rule, Don't Prep Plots, and Node-Based Scenario Design. I will summarize briefly here, but strongly advise you read the articles in full.
The Three Clue Rule
When you want your players to solve a puzzle or come to a specific conclusion, you should leave more than one path to success. Leaving too few could cause them to miss clues entirely - especially if they decide to take action in an unexpected way. Leaving three clues to a new storyline objective will increase their chances of succeeding on their own - very important when a quick, GM-intervention fix basically amounts to railroading.
Don't Prep Plots
Preparing a true-to-form storyline is the very definition of railroading. Instead of preparing a rigid plot, prepare pawns that embody the plot you want. Say I have a crime-boss villain who wants to take over a city. He has the ear of a dirty politician, several heavily-armed enforcers, and a group of engineers building an explosive device. The story is encapsulated in these pawns - my story is what happens when the players don't intervene, and the game is what happens when they do.
Node-Based Scenario Design
Even prepared story should allow the players to make choices that feel like they have impact. Creating a plot comprised of story "nodes" (e.g. "a confrontation with the engineers building the explosive") - each with clues pointing to other nodes - allows your story to be navigated naturally by the players. This article series is particularly long, so I won't summarize further here.
First, no plan survives contact with the enemy. The same is true of RPG detailed plots. You can try to make your players go a certain route: this is called rail roading and is generally viewed as a bad thing™. Even if you plan all possible moves, the players will chose one you did not think of.
Second, player agency matters. This means that the players must have an impact on the world. Their (in)actions should matter. They are the heroes of your tale, not actors to either support your favourite NPC(s) or witness the brilliance of events happening around them.
Finally, knowing what type of game everyone wants to play is essential: the same page tool is a good starting point to get discussion going as to what you all wish the game to be like.
As a side note, system matters so choosing a system (Cypher, Fate, Apocalypse World, D&D, etc…) should be obvious. But that requires you know about those choices in the first place. There are a few generic system (GURPS, Cypher, Fate, …) that can be made to fit any setting (though not necessarily any type of story). Or you can use no system at all2.
A good script can give you a good idea of what the major NPCs and their factions will do provided that the PCs do not mess with it. This is actually really useful. It allows you to work out how factions interacts, how they respond to changes of circumstances, and partition their knowledge.
A good script can serve as a good base for a lore document which can be given to the players to read. Or rather, different versions can be given to each players depending on what their character knows. It could be your "the story so far…" type of thing.
There is a fair amount of jargon in this answer (player agency, system matters, etc…) which you can search for on the wider net or on here for more detailed explanations.
1: A bold statement that require to be taken with a grain of salt. Some script can work for some games but there are mostly exceptions to the rule.
2: A somewhat controversial choice but the one I favour and have been using for years. Although I would only recommend this for experience GMs.
The difference between writing a novel and planning an RPG campaign is that in a novel, you control the protagonists. In an RPG, the protagonists do what they feel like. That means not every plot which works well for a novel will also work well for an RPG. Especially when that plot hinges on the protagonists making certain decisions. When you are a GM in an RPG session, you have to be aware that the player-characters will:
A good GM will do their best to accommodate such surprises and diverge from the plot by improvising. A well-planned campaign should be able to cope with that divergence and still move forward even if the players don't always do what you expected from them. Remember that being a GM is not about telling your story. It's about developing a story together with the players.
A test to see how well your plot works as an RPG is to imagine what happens when you replace the protagonist with someone with a completely opposite personality. Does it still progress? Does it still get resolved? (Not necessarily in the same way as you planned, but still in a meaningful way).
Novels and RPGs are very different medias that don't rely on the same narrative structure. Novels are a more passive one where the reader can't change the scenario (like in movies), and RPGs implies the player and should at least make him believe his choices have an impact on the scenario (like in video games). This is why your script will never become a good campaign, but it doesn't mean you can't use your script to enhance it.
Think about most of the Star Wars adaptations as a game (rpgs but also video games): most of them have a completely different scenario than the one of the movie but the setting is the same, and it makes these games greater. You can use your novel to build your setting:
it can be something that happened in the past (the story of their parents for example). It adds lore to your universe.
it can be something that is happening at the same time but won't come up soon. Having your script makes you able to know what is currently happening when the PCs will start to get interest in the problem.
