I've technically done online marketing since I was teenager, starting with referral marketing around 2010 and properly getting involved in the industry in 2013 with affiliate marketing.
Before that, I used to offer repair services for computers, phones, and general electronics to make extra money. At the time, I lived in a community that had a much wealthier neighbourhood a block away from the home my parents were renting, so I built up a reputation for being able to fix electronics. At first, the kids around there would pay me $50-$100 per device just to fix their computers and phones. Then, their parents found out and would pay me the same amount to fix their electronics. I was far cheaper than the actual businesses in the area and it was very simple for me, usually taking no more than 30 minutes each device.
This ended up escalating into me performing hardware and software modifications on not just phones and PCs, but other electronics such as PSPs. Before I knew it, I was getting paid to root Android phones and jailbreak iPhones as well since those were becoming more popular at the time.
I never set up an official business entity for it, but it's how I made money before this.