Before you start your body paragraphs, you have to establish a clear argumentative thesis statement as part of your introduction. Ask yourself what you are trying to prove about Hamlet's free choices? The answer to that question will be the focus of your paper and should help you keep the body of your essay on topic in order to prove something about the play.
In regards to your topic, here are a few questions you could ask yourself:
1. Does Hamlet actually have free choice, or is he a victim of fate?
2. Who or what influences Hamlet to make the choices that he does?
3. Does Hamlet think about the consequences of his choices? Does he over-think them?
4. What outside influences affect Hamlet's choices?
5. How does Hamlet react to the consequences of his choices?
6. How much of the resolution of the play is a result of Hamlet's free choices? What aspects are out of his control?
After you have done some brainstorming on questions like those posed above, then you can create your thesis and decide which choices/decisions made by Hamlet best support your thesis. Don't forget to use specific example and to explain HOW your quotes and paraphrases support your thesis. Good luck!