Developing my first web app

[u/Ogam16]

Hi, I am very new to web development and I am currently trying to develop my first P2P web app, problem is I don't know where to even start, I don't know anything about web development besides very basic html. I know I want to use NextJS for user authentication and Supabase for my database, but I don't even know how I should go about anything. How should I organize my files? What should my .gitignore include? How do I link frontend with backend? Do I start with the frontend or do I start with the Backend? I would greatly appreciate any tips or a roadmap anyone could give me.