How to structure a psychology essay at university level?

I've just finished AS year at college and we've been given the option to enter a competition at a local university. We have to write an essay based on any of the topics in the OCR spec for psychology and so I've chosen to do one on depression. I was thinking about writing about the different treatments for depression and whether drugs or CBT (or both) are most effective. I think this essay will be good practice as I want to study psychology at university however I haven't the faintest clue how to structure this. Do I have to also list the causes, symptoms etc of depression? As with only 1500 words, I don't think I'll have enough to write the causes as well as treatments because I want to include a study and then evaluate that. Any help will be greatly appreciated. Thanks! (This is almost my first post... not a clue what I'm doing!)