amysorto has given you an excellent example and some excellent advice. Of course you will have to write your own descriptive essay since it is your assignment, and I have included a short but helpful eNotes link on writing the perfect paragraph which you might find very helpful as you prepare to write.
The goal of descriptive writing is to appeal to the senses of your readers; you want them to feel as if they are really there, wherever that it is. If you live in Hong Kong or know it well, that is an excellent choice for you. If not, I would suggest writing about someplace you know well, even if it is less exotic, because you will have more to say. You will always write more effectively about something you know than if you have to rely on what other people say about it to write your own description.
Look around (or think about) the sights, smells, and sounds of a city. Consider landmarks (even if they are not famous ones) as well as the relatively unknown spots in this city. Tell us about the places where people gather as well as the places they avoid. Describe the things you appreciate most, remember most, and miss the most about the city when you leave it.
The key to this particular kind of descriptive writing is organization. You cannot have your readers bouncing all over the place, so choose some kind of spatial order (one side of town to the other, the outside of the buildings to the inside of them, the highest things to the lowest things) so your readers can move along with you smoothly. How you connect your descriptive ideas matters.
[Y]ou have to get from one point to another artfully and effectively. Try to vary how you move from one idea to another. Sometimes sequential ordering (first, next) works best, while other times you may want to find relationships (similarly, together with) or consequences (as a result, consequently) between your ideas.
This is just an idea, but why not try writing about your own city (someplace everyone in class is undoubtedly familiar with) in a unique enough way that no one will recognize it. In other words, choose the details that still describe your city but are lesser-known or at least less-recognizable. That idea may not meet the guidelines of the assignment, but it would be fun to try if it does.
The skyscapers tower over the city. The sounds of the subways fill the underground. Above, the smell of hot dogs comes from the many vendors. Everyone seems to be in a hurry, shuffling a long. There are so many tourists, which makes the streets overcroweded. Everyone and everything is so close to each other. Although, there are glamours parts to the city, and others not so much. At night, the city lights up like never before, especially at times square! This small island on the east coast has much life, it is incredible!
Manhattan, New York City, USA
I did NYC because thats the one I know most about. You have to give descriptions that make Hong Kong what Hong Kong is. Also, give geographical hints to help the reader guess that the city is Hong Kong.