How do I ensure my answer remains analytical rather than descriptive?

It is crucial that you choose a question which has scope for evaluation - perhaps assessing the impact or success of something - and debate. This way in answering the question, you will automatically be analysing rather than falling into heavy description. It also gives you chance to practice using academic authority, as you will start doing in University, to back up your points and to weigh up both sides of the argument before providing your eventual conclusion.