I have worked in admissions at a university that uses Common App, and I can tell you that the most important part of your essay is proving that you understand the program for which you are applying (and that you are a good fit).
It can be easy to think that the essay is meant to demonstrate your writing skills to the university. Don't get me wrong, it is essential to have crisp, clean work (edit, edit, edit), but rarely are the people reading an application looking at the quality of your writing. Your grades and test scores will show the readers that you are a worthy applicant, but your essay shows that you belong in their university.
Rather than focusing on the mechanics of the essay, it is much more important to articulate that you know the courses that you will be taking. Feel free to show which professor you would be interested in taking. The point of your essay is to demonstrate that you have done your homework and are ready to engage in the rigors of your degree.
Best wishes with the process! It should be fun, and hopefully, it will teach you about yourself. Please feel free to reach out to me if you have any questions.