I haven't had any stable job yet, I'm a full-time student right now. But when I was around 15, I earned my first money with an "odd job" I had (I don't know how to say it in English? Couldn't find a proper translation). Basically I helped an accountant with some tasks, sort out paperworks, overall bureaucracy. I worked for her for almost two years, sadly she moved back to Uruguay where she's from. It was a shame because she was considering to hire me as a secretary when I turned 18. But it was a great experience and I learned a lot such as payroll accounting.
Also, I offered some tutoring for Biology and Chemistry to high school students for a brief time.