These sections of the PMBOK (Project Management Book of Knowledge) form the basis of what you're looking for.
Part 1, Chapter 5 (Project Scope Management)
Section 5.2 (Collect Requirements) and its three subsections;
5.2.1 (Collect Requirements: Inputs)
5.2.2 (Collect Requirements: Tools and Techniques
5.2.3 (Collect Requirements: Outputs)
These are the steps of gathering requirements:
Ask Questions to domain experts
Analyze and observe the old process and opportunities to add features
Brainstorming is a must... if you put quality time on it, then you will ask questions and in return you will have answers. Make an activity or flow diagram based on what you have gathered... ...