There are many templates and formats people follow while writing a PRD (product requirement document), based on the industry or the market their product is catering towards.

Download this SaaS PRD template, here:

SaaS PRD template - Google Docs

The template has all the sections and placeholder texts helping you understand what each section is and how it has to be filled.

Here’s a good article which in which Manas explains how he formulates the PRD. This post and the template is a little modification to the same.
How I formulate and use my Product Requirement Docs