The usual format for a five paragraph essay would be a first paragraph setting out your thesis and introducing three supporting ideas. In the next three paragraphs, you would more fully explain your support and why it helps to prove your point. Your final paragraph would restate your argument and summarize how you have provided reasons for it.
You need to start by deciding what you want to say about Juliet, which may depend upon your assignment. Do you want to argue that Juliet was foolish to become involved with Romeo? Do you think she was brave and mature beyond her years in her handling of their affair? Do you want to argue that she was justified in rebelling from her parents' way of life and ideas for her future?
Once you decide your basic thesis and figure out the points you are going to make in support of your thesis, you are ready to begin writing.