Resources/Tips on building a website/app

I'm a 16 year old teenager working on a project that aims to give a voice to individuals with mental/physical illnesses and disabilities. And after having people fill out a survey asking what they think is the best way to give them a way to tell their story, they decided on a chat-based website/app where they can communicate with other people going through a shared experience of living with a health condition. So I've been looking at making my own website that does this but I've been struggling to find the right platform to work on. I was wondering if anyone could please guide me to articles/tips that could help me make this chat based website on my own? (I've coded a tiny bit before, but forgot everything - I am however, a quick learner and very technology literate). I would also like it to include a platform where people can upload pictures/videos etc. At the moment, I'm looking at starting with a mock-design of the app/website (app preferably). As I'm not looking for anyone to do it for me, I hope I can get these tips for free? Thank you!!!