It's different for everyone, what is important when writings stories, versus what you can do without. Everyone has different styles and different ways to go about writing, but this is some things I have picked up over my ten years of writing, and advice I have gotten from my editor once or twice.
Five senses
I tend to use this trick a lot when I am first opening a new scene or a new chapter, it helps the reader immerse themselves further into your books setting. Sometimes I get carried away with describing what is seen and ignore other ways in which one can describe a setting. Use the five senses to help you make it a more well-rounded experience. Don't just speak of the snowfall upon the village houses as your character walks, talk about the feeling of the cold biting at their nose. Or the smell of chocolate coming from the bakery nearby, the taste of peppermint in one's mouth as they sip a latte. The sound of laughter and music. Using all the sense, as we do in everyday life without realizing it, makes it feel more real.
I also use this when I am editing, and I feel things may be missing. Count off your five senses to help make the best picture for your readers.
Knowing your anatomy
The anatomy of the body, as well as the physiology, can go a long way when describing a character's feelings in a realistic manner. Just looking something us to help you better understand a gunshot wound, or hypothermia, makes incidents in your book so much more realistic and pleasing to hear. I work in the medical field and there is nothing that annoys me more than reading something that doesn't sound realistic when it comes to the human body. Knowing that the average human body can only go 7 days without water, or can only sustain consciousness for around two minutes underwater. That the human body can only lose 40% of there blood before needing a transfusion. Some of the symptoms of a concussion are nausea, dizziness, vomiting, and headache. Internal bleeding symptoms are abdomen rigidity, bruising of the site, dizziness, light headed, etc.
Knowing what your characters will experience after trauma or because of one makes it realistic and exciting. Not to mention it makes you look very smart.
Research
Just because you are making things up in fiction, doesn't mean things associated with that shouldn't be researched. Knowing more about what surrounds the thing you are creating makes it seem more real to those who read it. Makes them think that maybe this could be real, with all the detail and thought that is put into it. if you're going to make a hover car, maybe do some research on hovering mechanisms, or real cars and what you may use from them to create your own design. If you are going to make a new species of human sea creatures, research fish, and animals who live by both land and see, using them to help make them more real.
Make it seem as if your reader were to look outside right now, there is a possibility that your creation could be real, that your details connected it enough to reality to make it so.
Lay it out first
I started doing this recently because I was having difficulty with my editor on how I was describing certain backgrounds and scenes. Always lay it out first before throwing your characters into it. Write down details about it, draw it out (even if you aren't good), and make sure you add details you can use later. It makes it easier for you to imagine where you're characters are going, where they should hide or fight. Instead of just writing it as you go, because then you may be describing something that could never exist because it would be physically impossible given the architecture. (Yes, this is fiction, but connecting some realism makes it easier to understand.)
So your characters are just going to raid a simple warehouse, but then you start talking about offices and a balcony and I am completely lost. You are just writing it from your head and I, as a reader, will not know what you are seeing unless you tell me. What is the warehouse made of, what kind of supply is inside? Use the five senses to help me better acclimate to the surroundings. If you lay it out first you can make it easier for everyone else to see it.
Some of these may seem like common sense, but I like to be reminded of them. Use them as you will and if you feel I am missing something or you don't agree to, feel free to let me know!