Look at the Revision bite on Writing - Exam advice and types of writing tasks.
You must not be tempted to put in too much unnecessary description but select details which are needed to create the impression you want your reader to have.
Don't focus on something unless it is needed for plot, or to create a mood, or to capture atmosphere.
Look at these two descriptions of a room -
"The room was square with a window along one side. It had four chairs and a TV and video. There was a drinks cabinet and computer in the corner. The carpet was red and the ceiling cream."
"The room was brightly lit by a large window and housed several modern pieces of electrical equipment but the effect was softened by a drinks cabinet and a warm red carpet."
The first is a bit like a list isn't it? It tries to give us every piece of information even though they will probably not be needed again in the story.
The second is a bit better because it tries to give an overall impression of the room and also manages to hint at the character behind it - that is someone who likes technology but wants to be comfortable too.
Often describing a scene can be used to help portray the mood of a character, like in the following example.
"A wet, dull day greeted Mary as she stepped into the grey light. Men huddled by in drab wet coats or stood in dismal doorways waiting for a bus which never seemed to arrive."
What sort of a mood do you think Mary is in?
Now have a go at describing a scene which would match Mary on a happy day.
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