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I wish someone had assigned me to write a composition about music when I was in the ninth grade. There are so many ways to approach this assignment. One idea is to discuss musical genres, for example, classical, folk, blues, rock and roll, and so on. Another might be to briefly discuss the history of musical instruments. You could also take one particular piece of music you like and write about why you like it and how it makes you feel. Still another idea might be to discuss the difference in Eastern and Western music, since they are so very different.
No matter what subject you choose, be sure to have an introduction that has a thesis statement, a sentence that tells the reader what your main idea is. For example, you might say something like, "The history of musical instruments goes all the way back to ancient times, and musical instruments are still evolving in our times." This tells your reader what you will be covering in the composition. Then give your reader all the details that you need to explain your main idea. Finish up with a paragraph that wraps it all up and reminds your reader what your main idea is.
I have provided a link below about someone who spent most of his life writing essays on music. You might find it interesting to read.
You might want to first ask yourself whether you want to address the assignment as a music-maker or a music-listener/consumer. Do you have the technical knowledge that would allow you to discuss music theory, tonality, keys, history of instruments, genres of classical music, etc.? Or are you more comfortable assessing music's emotional effects on the listener? Or are you interested in the business end of music--where art and commodity meet? You might want to look at your major academic interest now and discuss its relationship to music. At the base of a good essay is an "A" idea, and that comes from looking closely at the range of directions the assignment allows and then following your personal interest inside that range. I once gave my students the assignment of writing about music, and a student wrote a brilliant paper on R.E.M.'s "Everybody Hurts" as an anti-suicide anthem for teens. This was an "A" idea.