New website - best practice for requirements specs?

Hey guys,

As a hobby freelancer I'm new to this. I've never had a non-technical client before explain to me what his future website is supposed to do.

A person wants me to make a website for him and he basically explained to me what's it about. However, he's not a technical person and he just doesn't understand what I need to know and how to properly describe/explain it to me.
When I ask him how a user is supposed to submit an entry to the website he told me "He fills out a form.", which is not really helping me. This was just an example, it goes on for other sections of the website as well which are a lot harder to explain.

The website will be aimed at a specific professional user demographic and I have no clue about their profession and how their industry works.

I tried to find some good Product Requirements Document templates on Google but none of them really seemed like they could help him understand how to write it so I can understand what he wants/needs.

Can somebody please give me a hint on how to deal with such non-technical clients?