You need to know: HTML, CSS, Javascipt, and Python, then pay a monthly or yearly fee to get a domain

Ok so here’s a “quick“ overview

Learn HTML, JS, and CSS (python is not required at all) and build out a UI; learn a backend library like expressjs (NodeJS) or flask (for Python) to make a rest API, and look into which database you want to use (SQL or other) and implement that in the API. Note that it may be difficult to migrate afterwards. After you’ve done that, connect the frontend and backend using http requests and stuff and consider using a framework/library to use components. After that you probably have the shell of a social media. Make updates to it until it’s solid, and then you can move on to hosting. Hosting has a few options: Vercel is pretty good for a frontend, but you’ll need to either self host or pay a hosting provider for the API. Now for buying a domain: Buying a domain usually costs between $5-20 a year and it depends on the registrar and domain. I suggest Namecheap for most domains, with Cloudflare for SSL unless you plan on multilevel subdomains (probably not). Understanding DNS is probably a good idea at this point. Now you can share your website with people and fix all the exploits, as well as moderate it using tools that you have to build.

TLDR: Don’t build a social media website as your first project