Seeking to make a dynamic website right from the scratch. This is going to be my very first project. How to begin ?

I'm not sure why you're getting downvotes exactly.

A basic tutorial on web development like Sleepless One suggested is definitely a good place to start, just to get a basic overview of what you're getting into. I personally learn best by doing rather than by learning. What I mean by that is if I sit down to try to learn... say... the C programming language, I'm probably not going to learn much from it, let alone retain it. But if I decide I want to write a game in C and start writing the game even from what little I know about C, I'll learn as I go. Not to say for me there's no benefit in a "learn C" tutorial, but if you're anything like me, I'd recommend switching to doing the specific website you have in mind as early as possible rather than trying to "learn web development" before switching to the project that is ultimately your end goal.

Beyond that, you'll want to avoid falling into a trap of doing what feels to you like it'll work rather than what's "best practices" for "the industry." So the other thing I think will benefit you searching-wise is to look for information about not just how to make it (technically) work but also how to do the thing you want to do "right."

At least that's my recommendation.

Beyond that, are there any existing websites that closely approximate what you have in mind for an end goal for your project? If so, could you share one? I think it might help us with more specific recommendations.

Please, whatever you eventually choose to do, make sure to continually reference this amazing website whenever you are implementing any interactable part.

https://cheatsheetseries.owasp.org/

It has cheat sheets for securely implementing everything from login forms, preventing common vulnerabilities (at least look at sheets for Top 10), forgoten password flows, storing passwprds and more.

From the top of my head, If you are building it from a scratch without a framework, you will definitely want to at least look into cheat sheets about input validation, injection prevention, password storage, session management, file upload and authorization with authentication.

They are not that long, and should prevent the most critical and common vulnerabilities you will probably have, where the prevention isn't too difficult, once you know about it.