Your own decision
It is up to you to decide whether to participate in research. It is your own choice. People participate on a voluntary basis. You are, therefore, never obliged to participate. You are allowed to refuse, even if your physician asks you to take part. You should therefore only decide once you know enough about the content and any risks and benefits of the research.
You can only participate in research as a research subject if you have agreed in writing (signature).
If you do not want to participate, you do not need to take any additional action. You do not have to sign anything. You do not need to explain why you do not want to participate. Are you a patient? If so, you will receive the normal treatment you would otherwise receive.