Before you start with the prototyping phase of the system, you will need to know exactly what your requirements are. There are many techniques for doing this and the ones you could choose depend on the budget for the project and how close your deadline is.

There are two types of requirements, Functional and Non-functional. Functional requirements look at how the system works, if it does what you would expect, if the feedback is good, and more. Non-functional requirements look at the environment, the user and the usability e.g. the users skills, frequency of use, learnability, effectiveness, memorability, accuracy and more.

Here are a few examples of my requirement gathering work:

Direct Observation:
I conducted a direct observation on users to find out how they went about buying a laptop online. This was to identify key features or names that the users were looking for.

Questionnaire:
A questionnaire was created online to provide me with data about the types of computers people will buy online.

Interview:
I asked sales consultants in different shops various questions to find out what the key selling points of laptops are and their general feelings and observations about the industry.

Fly on the wall:
I observed a customer buying a laptop in a store, listening in to the type of questions the user was asking as well as key selling points the sales consultant was saying.