Learn web development, learn linux. And most importantly be creative. For example, what will make your forum different from Dread, or what will make your market different from the other ones?
That's true, if I could make a site I would make a kink site with a bunch of chat categories for different kinks so degnerates can talk about feet and share foot pictures or affair chats where they can plan hook ups like Ashley Madison, then I'd lure in OF models to do cam shows which only members could watch but could donate money (like twitch) to get them to do random shit like blow bubbles out of their asses or something, it would be like an online strip club. That shit would make bank.
It is not as hard to make as you think, but to be good and survive is the difficulty.
Do not bother with HTML and CSS besides the basics. Anything advanced should be developed with a local LLM to frame the website design, then have professional actually make it look nice.
You want market or just website then you need to focus on the "stack". LAMP = Linux, Apache, Mysql, PHP. This covers the system, the database, the web server, and the backend language. You need to find a stack that works for you and know everything about it. I would recommend linux, nginx, postgres, and the backend of your choice (php, golang, or rust). I would not use nodejs or python as they have so many problems with backdoored libraries the past few years.
You do not need a lot of money unless you outsource everything. For web vulnerabilities there are a lot but the basics are not hard to get decent at. Things such as SQL Injection, XSS, and a few other web vulnerabilities already have built-in filtering or protections in languages like PHP (if you read the documentation). You will want someone to pentest your application as well but you can find reasonable people for an okay price.
The simple story is pick a stack and spend some months learning it. There is no set time of when you can do it and when you can't. Some people are good at this, and some are not.
An Edit: I want to make clear that if you want to make it successful in this type of world you need to own your stack and be the only one with access to it. If you outsource any part of that stack to someone else and you don't understand what is going on with that part of the stack you will get your system fucked.
HTML/CSS, PHP, GoLang, Lua, SQL variants. Collectively for +5 years if you want to be independent
Otherwise hire people to do work for you, if you got the capital for it.