Start by making a list of why you are going to nursing school and what you hope to accomplilsh. If you base your answer on anything other than that, you will be setting yourself up for failure. If it doesn't come from that list, it will sound false. Also, you are likely to be asked about it in your interview -- and your interview answers won't ring true if they are not genuine.
So ... I suggest you jot down a few ideas from your heart and share them with us. Then we can give you some feedback on how to state them.