If you're under school-leaving age, there are some jobs you're not allowed to do. There are also limits on the number of hours you can work per day and the actual hours you work. (e.g. you can't work before 7.00am or after 7.00pm.). The rules for 13 & 14-year olds are different for 15 year-olds (and also 16 year-olds before their school-leaving date). You also have to have a permit from your local eductation authority.
The details are here:
http://www.direct.gov.uk/Parents/FamilyIssuesA ndTheLaw/ChildrensRights/ChildrensRightsArticl es/fs/en?CONTENT_ID=4002945&chk=AaZA0p
Most big companies (e.g. national chain stores) don't employ people under 16 years of age. Your best chance of finding a part-time job is with an independent trader (such as the owner of a local shop or a market trader). Unfortunately, these type of jobs are rarely advertised. All you can do is trawl around shops, market stalls and other businesses hoping that someone needs some extra help.
Good luck!
Chris
PS: I note, from your post in 'AB Suggestions', that one of your questions, in I&T, has gone missing. Your best chance of getting a response from the editor is to use the 'Contact Us' link, at the bottom of each page, rather than posting in ABS. If your question about laptops was simply seeking a reliable source of cheap laptops, see here:
http://www.accomputerwarehouse.com/products/No tebooks