How do you decide what to include in your description of a scene?
Remember that you're in a character's viewpoint. Ask yourself what is important to that character.
A cop entering a room where a gunman may be hidden is seeing different things than an interior designer who enters a room a rival has just decorated. The cop doesn't give a damn about the charming shade of blue in the wallpaper, but he'll notice the large pieces of furniture someone could be behind, the amount of light and shade in the room that makes seeing movement tricky, and the possible exits.
At the same time, the character will be aware of the sounds and smells in the room-- the faint smell of gun oil, the Chanel No. 5 of the wealthy woman who owns the home, the tap of the nails as a toy poodle moves across the oak floor, and the slight rustle of something moving behind a curtain.
With just the right specific touches, the room will come alive for the reader and at the same time you're building tension and giving character details, and you're not stopping the action with too much description.