As a Scrum Master, one of the responsibilities is to help the team pulling up the backlog for a sprint which is already prioritized and pulling items which are placed on the top (sorted as per the priority). Once, the team is able to identify items that can add value from the pile of requirements, the Scrum Master can help them (Product Owner + Development Team) to convert them into good user stories (if it is not already).
A good user story is one which includes a description and has acceptance criteria defined. It should be a piece which can be delivered in a sprint and entails minimum dependencies. The team should be able to develop and test within the boundaries of the sprint along with providing the estimates. In short, good user stories follow the INVEST principle.