Writing a Spanish essay is like studying a new language, especially if you have just enrolled for a course in the language. The task consists of writing about a language you have probably not learned before. The ideal way to start is to write several practice essays before you attempt the main essay for submission. This can be done while you are learning grammar, sentence structure, and building up your knowledge of vocabulary.
Choosing essay topics is not necessary while you are learning about the subject. Your tutor would be able to offer you help on this. There is a need to write on topics that would enable you to write fluidly and fast. This takes a bit of time till you are well versed with the intricacies of writing Spanish. In fact, the essays you write on various topics for practice allows you to learn faster. Your tutors would be guiding you in the direction you should be taking.