How to chose design model to implement

Question

Hi
Please guide to solve my following question
Basically i want to know how can i start.
There is a company called ABC, they want to have website which displays the List Catalogs, user can select any catalog which will display products in that particular catalog, Again user can select any product and chose its attributes.
1) Write What design model do you adopt to implement this?
2) After product went on to live, client wants to add discounts feature to department and products. How do you implement this, explain with the change model.
Thanks in advance