how exactly should I answer this question comparing plant and animal cells?

Firsty, read the question carefully, making sure to underline any key points so that you understand exactly what the question is asking. Look at the marks awarded for the question, this should give you some indication of the level of detail required.
Then, once you are clear on the question, begin to arrange your thoughts. In this instance you should start by making a mental list of the similarities between plant and animal cells. Examples of this include the presence of organelles such as mitochondria and nuclei, similarity in overall size and the fact that both are eukaryotic cells. Then, make a mental list of the differences between the cells such as the absence/presence of chloroplasts, plasmodesmata and cell walls. Finally write concisely and to the point in a clear structure in order to allow marks to be awarded easily.