Note, I said "almost no monetary investment."
I did things like buy a domain name, pay for cheap web hosting, etc. I was already knew the basics of coding, and some understanding of marketing. My biggest investment was a $97 eBook about marketing. I found one idea in that book that I used to do some successful SEO. This was circa 2003, when SEO was pretty much as easy as picking fruit from a tree, if you knew the right things. Anyways, that's how I got things launched. I also bought and read quite a few business books, which certainly weren't free.
This type of investment was very do-able for most middle class kids. Many of whom, at the time, could afford their own home built computers.
Also -- what I started with is very much not where I ended up, though there is certainly some relationship to it. For example, SEO is still an important part of the business, but the SEO strategy is much more about human relationships and service, than it was about "gaming the system," which was my perspective as a 21 year old.