It’s a combination of things, but most sites you’ve heard of now are either started by someone who can do the work, or they hire someone for a simplified “1.0” version of the idea to see if it really is a money maker.
I read somewhere that digg was started by hiring a developer for $1800 (maybe it was $1200?) to do a super simple version of the site. Facebook I believe was developed by the people who started it in college, CNET I have no idea but for some reason I thought was part of a larger news organization.
If you have an idea you believe in, and it’s purely a website that is needed to get it kicked off, then really you could probably find someone with the skills and advice you need to do a basic, get the job done well, nicely designed, website for $2,000 – $5,000. Which compared to other business types, is a tiny tiny amount. As mentioned, if someone believes in your idea as much as you do, then maybe part of that could be equity in the company.
Is that first version going to have absolutely everything you’re dreaming of? Probably not, but the focus is on executing the core of your idea well enough to see if it strikes a cord with your audience. Give them a good experience, get the word out, and you’ll have a good feel for whether your idea fills a niche.
In all honesty, a lot of people have great ideas for websites, just like a lot of people have great ideas for restaurants or bars. In both cases the actual skill of starting a business is in executing the idea in a way people will enjoy or find useful. The idea is unfortunately overrated. No one expects to set up a bar, a sporting goods store, or even a lemonade stand for free, somehow the web is the only place where the no-money-down business is rumored to exist.
The good news is that a website can be started for a tiny percentage of what you would need to even get the doors open for a restaurant or bar, you just have to be willing to either learn to do the work well, or hire someone who can.
Good luck with your venture and let us know if we can help.