Start by defining what kind of reservation system you want to write and what capabilities the system must have. For example

1. Goal is to design a Hotel Room Reservation System
2. The system must provide the capability to place new reservations
3. The system must provide the capability to cancel reservations
4. The system must provide a way to lookup available rooms based on date, room size, etc
5. The system must provide a way to view/lookup reserved rooms by guest name, date, etc