Made my first dollar online 15 yrs ago with a pokemon site I started in middle school. Used Microsoft's LinkExchange to get ads to push on my site.
LinkExchange - Wikipedia, the free encyclopedia
Eventually I got banned for having a low quality site. I think advertisers weren't getting much ROI from a bunch of 10-13 yr olds (this was well before shopping online was mainstream) and let's just say as a 12 yr old myself I wasn't strictly adhering to any "best practices" in terms of ad placement.
I then signed up for the now long-defunct AllClicks, which literally gave you a URL and paid 3 cents per click no matter how you got someone to click on the link. You were told to make up whatever anchor text or link whatever picture you wanted, if you got the click, you got 3 cents.
I actually made some decent coin (for that age) with giant text links that said "free pokemon cards" that in all likelihood probably linked to male enhancement products or porn. Fuck a quality score, lol.