In filling out the forms and essay, please keep in mind:
The application form and essay must be completed in English. It can be typed or printed, but the writing must be clear and legible. The essay must be typed and attached to the form.
Recommendation letters must also be in English. If the person writing your recommendation does not speak English, the best thing would be to have the letter professionally translated, with an official translator's stamp or seal. If that is not an option, then you should translate it into English. The recommendation letters should be printed on official letterhead that shows the name of the school or office, and should be signed and sealed, and made to look as official as possible.
The essay is extremely important to your success. It is your best opportunity to express your personality and individuality, while giving some personal background and explanation as to why this program is a good opportunity for you. The essay should clearly state your academic objectives and why it is important that you study in the USA. It should also make clear what type of program you want to pursue in the USA, how this program relates to your current studies, and how it will help you in your future objectives.
Regarding field of study: You can request to study within your current major, or if you can try to study something new in the USA. Anything is possible, but you must be able to express your reasons clearly. For example:
You are currently a Biology student. You like Biology, but you really have a strong interest in Marine Biology, specifically. In your application and essay, you should explain that your school in China doesn't offer Marine Biology courses and you have never lived near the ocean, so you hope to go to California to study Marine Biology and get both classroom and hands-on experience in this field. Marine Biology is not your current area of study, but you can see that there is a natural progression from your current field to the program in the USA.
You are a Literature student, majoring in British Literature. You like British Literature a lot, but you have always had an interest in American Literature. The best place to study American literature is probably the USA, so this program would give you the opportunity to study the literature and experience the culture. Going from British Literature to American Literature is not a very big change, but it makes sense that you would want to pursue that program in the United States.
You are currently majoring in International Politics. But, recently your ideas have changed and you really want to study Dance. Good luck. You will have to present a very strong argument and some very good reasons for such a big change. We are not saying that it is impossible, but it will be up to you in your essay to clearly express your thoughts and reasons.