Another popular job, particularly for girls, is helping out (by sweeping up, etc) in hairdressers' salons. The jobs are rarely advertised. You'll have to phone all of the hairdressers in your area to see if they require help. (NB: The pay in these jobs is often very poor).
Caf�s sometimes seek young staff to clear tables, wash up, etc. Once again, jobs are rarely advertised; you need to phone around.
Corner shops, and other retailers, sometimes take on young people to stock shelves or (occasionally) serve on the tills. Yet again, it's best to ask around.
Lastly, there's nothing to stop you setting up your own business. Car-washing and pet-walking are among the most popular but you need to 'examine the market' and decide where your talents lie. (I was writing for the local press at the age of 14. Later, when I was teaching, one of my 14-year-old pupils was earning more than I was, through writing regular columns in two computer magazines).
Chris