User Stories are small sentences highlighting the user's motivations and goals. They are part of the Agile process and help you think about what the user/customer wants.
They can be written as follows: "As a [type of user], I want [feature] so that I can [goal/benefits]."
and you should ask the stakeholders to write them. Once written, all the stories can be discussed and ranked in order of priority.
You can start with a wide user stories ("epics") and then go more into precise stories focusing on functionalities, details. The whole bunch of stories somehow replaces a list of requirements.