First you have to know what you need the reader to get from the setting. What's important? If it's a crime scene then every detail about the setting may be important. If it's lovers on a walk then only the general atomosphere may be needed. Once you've figured that out then you can figure out the angle you want to take on the setting description.
If your book is first person then of course you are taking it from the character's view. As cheesemaster mentioned you would notice what the main character would notice. If it's familiar territory you might only notice what's different. If you are coming in from the cold, anything warm might be important. First person will also give you some of the main character's personality. Harry's snark in the aforementioned "slobquake" is both descriptive of the room and of Harry himself. If your character is very observant, or snarky, or in tune with nature, or happy or sad or a hundred other things, these could all affect how he or she describes the setting.
If you are writing in the third person then you have other avenues available. You can simply describe the setting that you see or you can use anyone or anything's perspective. Robert Jordan started many (maybe all) of his books talking about the wind, where it blew and what it crossed, going from the general to the specific.
What Paynesgrey said is very good. A room with clothes thrown on the floow may just mean someone came in late last night and hasn't picked up yet. A room with clothes thrown on the floor, a musty scent of unwashed socks, and the sound of an old pizza crust crunching under your foot is indicative of a much greater degree of sloth. Seeing a cat in someone's house and smelling a cat in someone's house are two very different things.
edited for typos