You'll find it hard to get a job during the week because you're only allowed to work a maximum of 2 hours per day, Monday to Friday, during term time. You're not allowed to work after 7.00pm., even at weekends or during school holidays:
http://www.direct.gov.uk/en/Parents/FamilyIssu esAndTheLaw/ChildrensRights/DG_4002945
You can work up to 8 hours on a Saturday but, because you can't be available at the start of the day, you'll still find it hard to get a job.
From your screen name, I assume that you're a girl. Probably the most common type of work undertaken by schoolgirls is assisting in a hairdressing salon. (That's not sexist - just a statement of fact). I suggest getting the Yellow Pages out and phoning all the hairdressers in your area; you might be lucky.
Otherwise, it might be worth asking local market traders if they need any help on Saturdays.
Chris