Need Help with My First Frontend Developer Internship

I recently landed a frontend developer internship. My company asked me to study an existing website (like an e-commerce site) and create wireframe of the site and Documentation about it and features and how I am gonna implement it

The problem is... I have no idea where to start. i am good with HTML/CSS/JS, but this is my first time working on something this structured at a big level

Could anyone guide me on how to approach this project ??