RE: Beginer who try make sci fi short story

#2
I'll mostly address (1), since that's my (self-proclaimed) area of expertise, then lead into a bit of general advice.

There are two ways you can go. Realistic or fictional. I prefer writing realistic, but fictional is more fun/accessible to read and easier to write.
Realistic means that your spaceships obey the laws of physics. That means they carry fuel, speed up or slow down according to acceleration, and usually don't have wings for aerodynamic reasons.
Fictional means they act like planes floating in 3D space. They care not for things like gravity, micro meteoroids only exist in asteroid fields, and they are able to achieve orbit using only their basic ship-mounted engines. Fuel usage and travel times are often undefined.

The reason why you should think about these first is because it will affect all of your character and ship design. A realistic space station would, if the universe lacks gravity technology, either exist in 0-G or spin to imitate gravity.

Once you've decided that, approach this like a normal combat, but placed in the situation of a space station assault. Which side has larger forces? Which side has better equipment? Which side can use the environment better? And then apply this to the situation - maybe the attacking side has stealth tech on their ships which gives them the advantage. Or perhaps the defenders have laid mines in an area the attackers didn't know about and they get caught in them while advancing.

Tactics and so on are more difficult, but these can often be hand-waved, especially in the context of a short story, unless your main character has a leadership role in the assault. The good thing about sci-fi is that it's very hard to get a complete view of what's happening - information is valuable but not readily accessible.

I'll move onto general tips. You should have some idea of the process to take when designing the battle itself, and if there's anything you don't know, research it. I'll instead suggest more about the approach you should take to designing the flow of the story itself. This may be partially in response to your other questions, but I'm not sure what you were asking so I'll just say it's general.

Focus the story around characters and their reactions to events. You can certainly have sections of the story which diverge from this, but they should be short, and in a short story they should be reminisce, if they exist at all. In a short story, you have only a limited amount of time to show everything you want, and more than the world, more than the combat, you should want to show us the characters.

More than many genres, sci-fi - or good sci-fi, at least - is about humanity.
My (Science) Fiction - Desolate Stars.
And historical - Out of the Motherland.
Go to a new fiction. Review it. Help the little guys.