Don't worry. The university will ask you to write on some subject whether you like it or not. This is of course you want to enter their campus.

What do you mean? Are you talking about an Admissions letter to a university? If so , have a look here http://www.italki.com/knowledge/page/Sample%20of%20an%20admissions%20letter%20for%20an%20overseas%20university%20or%20institute.htm