DO use what you know about writing in English.
Let's start with the don't. Translation is a highly specialized discipline that requires significant training. Chances are if you are still learning a language, you aren't going to do a very good job at translating. So don't ever draft your work in English and then try to translate it; it will be a mess.
Do start with the Spanish you already know:
- Write a good introduction with a catchy hook that makes the reader want to know more. Let the reader know what to expect in the rest of the essay (and in what order to expect it). Note: it's a good idea to re-visit your introduction and re-write it at the very end of the process since the order of your ideas might change as you write.
- Develop a thesis. Assert something. Then support it with specific data, facts, examples.
In a class, the writing assignment will be geared toward your level so that you should be able to successfully complete it by using mostly Spanish that you do know and/or have studied. You might feel frustrated that in Spanish your writing makes you seem like you're no smarter than a fifth grader, but that's still a lot better than a poorly done and mostly incoherent translation that makes it seem like you have no control over the language at all.
Save your sophisticated skills for looking up a word here and there in the dictionary (not every word! and be very careful to check various meanings, changing forms, and important contexts--the dictionary can cause a lot of the same problems as translations do). Use circumlocution--describe what it is you don't know how to say in Spanish using things you DO know how to say in Spanish.