If you are an aspiring composer or arranger, you should train yourself to listen to the various instruments that make up a complete song or music piece. Learn to HEAR the different instruments such as the piano accompaniment, the bass line and the rhythm. Ask youself these questions: What instruments are used? When do the instruments play a certain phrase in a composition? Which instrument is used to play a certain part of the composition, and why?
Here's a tip. Every musical instrument is played differently in terms of technique and style. In order to make your MIDI instrument sound as rich and authentic as possible, you should play the instrument in the same way as its classical counterpart. For instance, when playing the bass guitar on your keyboard, you should try to emulate a real bassist.
Now that you have the basics, try the examples above. Listen to them and get yourself familiarized with how various MIDI instruments are played. These are common MIDI instruments found in music pieces.