My first "business" was in school, I use to buy loads of sweets in the morning and would be like a walking tuck shop in school.

We had a really good little newsagent who ordered some great sweets - I remember it well! hehe
Some of my best profits ever made really... I use to pay 15p for a pack of these super hot Gobstoppers, and people in school bought them for £1. (Yes I know it sounds really bad taking their dinner money for a pack of Gobstoppers lol

)
Then again in school, I had a stint in selling games and application for computers... This was around 1993-5, before the internet really, so passing disks (and tapes!!! - remember those! lol) around the school playground was still the done thing, but would be massively frowned upon these days.
I've been into computer repair and support since....
When I was 16 - just left school, going into 6th form, I started buying motorbikes and doing them up and reselling them on again, and not long after (once I acquired enough space) I started with cars from the auctions too, earning a pretty good side income.