Pick out the contrasting sounds of an orchestra, through listening to classical music on a radio, CD player or iPod. The woodwind section of an orchestra is softer than the brass section. The string section is really the emotional heart of an orchestra. Percussion is needed for dramatic and powerful climaxes.
Read about influential composers and the instruments of the orchestra. Learn how particular instruments have been dominant in certain pieces of music, created by some of history's important composers.
Go to a concert. Listen carefully to all the nuances of the music and watch how a conductor brings in each instrument or section. Try and conduct the orchestra in your own mind, deciding when you think a certain part of the orchestra should be utilized.
Follow a piece of music you are unfamiliar with and try and predict where certain instruments are likely to come in. For a crescendo, for instance, a gentle flute would only really be appropriate as a gentle ending to a crescendo, or as a lead-up to it, rather than being part of the crescendo itself.
Select each instrument you can definitely distinguish from another when listening to classical music. This can take time, but it will allow you to be more cognizant with the workings of an orchestra both individually and as a whole.