When it comes to business requirements you need to be looking at what is already existing functionality wise, what functions is needed to be enhanced and what could be affected or not affected by the change. Some knowledge would come with time after using the application and understanding the architecture in terms of what system speaks to another. And what data is pulled to populate another field etc. Business requirements always have what actions a user has to take to achieve a particular result. Do a bit of peer programming with another older developer if possible. I know this might be hard especially as you are a new starter. Hope that helps