First and foremost, you got to ask yourself, what importance is the room or setting to the story or characters? If the room is only there for a brief passing scene, it may just suffice to say "so-and-so went into the broom closet. It was dark, cramped, and loaded with brooms." That may be all you need.
For more significant settings, where you really do want to paint the picture in the readers' heads and firmly establish a sense of space or ambiance, then of course you'll want to dig into more details. The key issue here is that you don't want to overdo it. Unless you're typing out an architectural report or something, there's no real need for a reader to understand the full dimensions of the space, or what the composition of the walls are, or anything technical like that. You will want to cover the overall impression of space, color, mood, atmosphere, furnishings, props, and anything else, as long as it's distinctive, relevant, and contributes to the story or image in some way.
The objects in a room - furniture and stuff - may or may not factor into your scene. If you say that people are in the living room, chances are that the reader will automatically populate the room with their own idea of what a living room will have: likely a sofa, a TV, etc. So there may not be a need to describe what furniture is in that room, especially if such furnishings are not going to be actively used. On the other hand, if the characters are going to use something, it may be necessary to establish such things early in the scene, so the reader can understand that the given thing exists and the characters aren't just making it materialize. For example, if characters are in a room with a gun on the table, and one of the characters grabs the gun and uses it, it'll help to explain right away that there is indeed a gun on the table. Otherwise, it'll sound like the gun just magically appeared on the table. It may not be necessary for some things (grabbing a knife from the kitchen would be self-explanatory), but this kind of thing should be set up for everything else that isn't so obvious.
You'll also want to describe things if they're not usually associated with a given place. For example, in Robert Heinlein's Stranger in a Strange Land, some living rooms had expensive lawns in them, with actual soil and grass growing in the middle of the room. It was important to describe them, because the characters used the indoor lawns, and even commented on them. Later in the story, one of these lawns was ruined. It serviced the story (plus, an average reader will not associate grass in a common living room). For sci-fi and fantasy stories, where settings are imagined and re-imagined more vividly, more description may be necessary to paint a picture of a futuristic or otherworldly setting in the readers' heads.
Another thing to consider will be what the items of a room, or its decor and layout, says about the characters. If the room is messy, you can conclude that the character is disorganized, and thus you find another way to show a character trait. Or, if the room has expensive art, you can infer that a character has refined tastes (or maybe he just pretends to). The possibilities on this level are limitless; if a detail is relevant to a character, you will want to capitalize on it.
In the end, however, all of this will depend on your own personal writing style. Different writers will write settings in different ways. Wordy writers like Stephen King or JRR Tolkein could spend pages and pages talking about the stuff in their rooms; James Patterson never seems to describe any of his settings, especially if they're common places. Really, the best advice I could give is to simply approach the scene naturally, and write out the first things that come to mind. If nothing comes to mind, just proceed with the scene in the given setting with sparse details; chances are that you don't need details anyway. If you're compelled to say more about the setting, then try indulging in such details as your imagination allows, and see what comes out. It should be a natural occurrence; if you're stuck on describing a place, it might be best to just skip ahead, write the next scene, and go back in the rewriting session to see if you really need to add anything more.
And when it comes to your own writing style, there is no set way to describe a place. It's not like you go through a room step-by-step to introduce the walls, floors, furnishings, etc to a reader. If anything, this will come off as dry, long, and uninteresting. If you have to explain every little thing about a room, it would be better to break up the exposition with action or dialogue; you have to keep the story moving, and lingering on interior design may stall plot progression. You also don't want to make the language describing the room overly dry or overly flowery; just use your natural narration.
In summary...
Do:
- Keep it simple.
- Talk about colors, patterns, decor, and unique architectural details, if they're relevant.
- Talk about furnishings and props, especially if characters use them.
- Talk about anything in the room if it reveals something about the characters within.
- Talk about space.
- Talk about unique details that readers may not usually associate with a given place (especially for sci-fi and fantasy works, where the settings are purposefully different anyway).
- Describe it naturally with your own personal writing style and sensibilities.
- Get technical or overly-explicit.
- Divulge in unnecessary details.
- Tell about room's atmosphere or impression; show it instead.
- Overthink or overdo things.
- Dump details in one long paragraph.
- Describe things in a dull, dry, choppy, or uninteresting manner; use your natural narrative voice.
- Describe things that the reader will already assume for a given place, especially if such things don't contribute to the story.
From Rider of the White Horse, Chapter 25
I wrote this story as far back as high school; I've always felt this was a very amateurish story with a weak writing style, but it's serviceable and got the job done. The description here is pretty bland, doesn't say much, and quite understated.
Kurt walked towards the old man, and he followed him through the ruins of Tokyo to a squat abandoned building a quarter of a mile away. There, the old man led Kurt into a relatively clean room with cupboards, a single mattress on the floor, and a low table. The old man lit a candle that was on the table with a makeshift lighter. The candle illuminated the room, revealing the old man’s face to Kurt.
The old man went to the corner of the room, where a tub of water sat idle. Kurt noticed that the man rigged a purification system over the tub, allowing him access to relatively clean water. The man took some water and some leaves he had stored in a cupboard. Then he prepared two cups of tea, working diligently with trembling old hands. Kurt sat at the low table and watched as the old man prepared the tea, observing the man’s technique as he mixed ingredients and stirred them in wooden cups.
From Perfectly Inhuman, Chapter 3
This is one of my most recent works. I did take the time to describe this area in bigger detail, to give the reader a lavish and futuristic picture. It reflects on the power and wealth of the Mayor and his government. Hopefully, you'll get the impression of wide-open spaces, luxury, and cleanliness.
At the topmost floor, the city became a mere map beneath Mary. The doors opened, and the guards pushed her out.
She found herself in a large lobby. The floors were made of colored tiles arranged in jagged patterns, and the walls were made of glass, revealing additional views of the city and the mountains to the east. A frosted glass partition separated the lobby from a private office. Silk banners hung from the ceiling. Polished stone pedestals held golden and silver statues portraying nude men and women. Everything in the room was rich and lavish; Mary found herself awed, and envious that she never had a place so luxurious.
The guards guided her through a set of glass doors in the frosted glass partition, and they passed into a wide open office space. The office looked much like the lobby, only instead of statues and banners there were holograms and display screens. Contrasting with the bright floors and the bright exterior view, there was a black desk on one end of the room, made of a rare dark organic wood.
From Ouroboros: Demon-Blood, Chapter 11
This is one story I've worked with on and off; I've been a little wordier with this series of stories than with most others, to try and immerse the audience in a more detailed fantasy world. Hopefully, you'll get the feeling of seeing something different and fantastic (and possibly wicked) with this segment. Note that the term Svartálfar comes from ancient Norse myth, referring to a race of Dark Elves.
In the middle of the woods, the Svartálfar had constructed a large settlement. It was surrounded by a thick wooden wall, studded with huge wooden thorns and metal spikes. The area around the wall was cleared of all vegetation, so that it could not be scaled with nearby trees. There were trees on the other side of the wall, which had platforms and turrets for guards to stand watch on. The settlement’s gate was a thick wooden door with iron supports; it swung open for us as we approached.
Inside the settlement, the Svartálfar used most of the trees as buildings; they were all hollowed-out to serve as homes, stairwells, storage, and stores. They also had small wooden shacks and huts in between the trees. Some buildings were also constructed on the sides of trees and on their branches. There were scores of elves bustling around, trading with their craftsmen, mentoring their children, and practicing with their weapons. When I entered the town, they all stopped to gawk at me; I met their gazes with my own look of contempt.
In the middle of the settlement, there was a larger tree, surrounded by a wooden wall with turrets all along it and a single gateway. I was led through the gate toward the base of the tree, which had an expansive hall jutting out of its bark. Passing into the hall, I stepped across a polished stone floor; the hall’s curved walls were ornately carved with elfin runes and mosaics. Twisted pillars held up the ceiling. At the end of the hall was the throne of the Svartálfar king, Lord Hygric. It was a large throne ordained with pieces of gold, silver, gems, and there were skulls hanging above it.
