To write effective descriptions you have to observe things around you and remember them so you can see them in your mind.
Always try to experience the whole picture in your mind using all your senses, what you can -
You don't experience the world just through your sight, why should your reader?
Even when describing a totally made-up setting or type of character such as an alien or fantasy monster you need to base your description on recognisable terms.
Have a look at the following extract and see if you can pick out which senses are used to describe the scene -
"He walked into the classroom and crossed to his desk. The floor was worn where years of boys had kicked and scuffed in their rush to escape. A damp, sickly-sweet aroma arose from the pile of jackets crushed in one corner and a blind tap, tapped against the window where a stone or ball had cracked the glass. Mr Jones rubbed his hands across the smooth polished top and examined his finger tips for dust."
Video clip: Descriptive writing
Don't rush your description, take time to use all you senses and let your reader "see" the whole scene.