Philosophy deals in reason, logic and ideas on existence. There are no wrong answers in a philosophic debate as long as you can sufficiently support your ideas with statements of reason--this is why law and philosophy go hand in hand. Both law and philosophy seek to interpret ideas, such as rules in the case of law, with reason-based arguments. When writing a philosophy essay, you simply need to follow some simple guidelines and back up your ideas with reason.
Philosophy essays are different from essays in all other fields. A number of years ago, some psychologists did an experiment to test reasoning ability. They found that people from most professions did equally badly; only philosophers did well. This probably indicates that the psychologists had taken their criteria for good reasoning from philosophers; it certainly indicates that philosophical reasoning differs from that in most other fields.
This means that the most important thing you should learn in this course is how to think and write philosophically. These skills can be learned, and anyone at Cambridge is capable of learning them. However, unless you have done philosophy before, you do not already have them, as they can only be learned by practice. That means that your first essay is likely to be quite bad (so don't worry if it is) and that it is absolutely essential that you do essays over the course of the year. Native wit will not get you through, and it is not possible to cram the subject in the last couple of weeks before the exam.
Your essays should be grammatical and correctly spelled, but I am not going to spend time in supervisions on basic literacy. If you have a spell checker, use it, because spelling errors that a spell check would have caught look particularly stupid. You should add the correct spelling of technical philosophical terms to your custom dictionary, if you have one. Grammar checkers tend to be of little use, and are only worth using if your grammar is particularly bad. To write a good essay, you need to have a clear sense, from the beginning, of what end you are trying to achieve. Your argument should develop from paragraph to paragraph, usually beginning with an introduction that tells the reader where you plan to go and how you intend to get there. It is generally best to write this last, since you won't know exactly where your essay is going until it is finished. It is important to remember you are not writing a mystery novel - let the reader know how it is going to turn out before they get to the end. This will allow them to assess the strength of your case as they go along. You should conclude your essay with a final paragraph that draws the threads together. Strong essays have coherent beginnings, middles and ends.