Jobs for teenagers are few and far between and hardly ever advertised. Start by making sure that you're aware of the rules which people employing young people need to observe:
http://www.direct.gov...entsRights/DG_4002945
Then you're probably going to have to start calling on traders (or, at least, phoning them up) to ask if they've got anything available. The obvious place to start is with newsagents but also keep an eye on your local free newspapers because they often advertise for people to deliver around local areas. (Even if they're not currently seeking anyone for your area it might be worth seeing if you can get your name on a list for future employment).
Some young people (particularly, but not solely, girls) manage to find work in hairdressers' shops. Again, the jobs are rarely advertised. You'll need to ask.
Market traders will sometimes employ young people but that would be at weekends, rather than after school. Also try independent convenience stores. (Big companies rarely take on young people).
Remember that the rules on employment (linked to above) don't apply if you're self-employed. Some young people make money by offering ironing, dog-walking or car-washing services. Perhaps you've got some special skills you can offer, or things you could sell on eBay (I remember reading of an 8 year old who was running a thriving antiques business. A 7 year old artist in Norfolk recently sold some of his paintings for £130,000 in a few minutes. When I was teaching, one of my 14-year-old pupils was earning more than I was through writing for several computing magazines).
Good luck!
Chris