At 14, the maximum time you can work on a Saturday is 5 hours. You can't work before 7.00am or after 7.00pm. You also need a permit from your local education authority.
The fact that you can only work for 5 hours means that many employers won't be interested in employing you. (The limit goes up to 8 hours when you're 15). Additionally, big firms are not usually interested in employing school-age people. You're going to have to find a small, independent employer to take you on.
Finding a job will be hard. Girls sometimes find work helping out at peak times in local hairdressers. (I'm not being sexist when I say that it's girls who do this work, but I've never known a boy to do it!). The work is boring (sweeping the floor, etc) and the pay is usually poor but (if you're female or if you're male and determined to 'break the mould') it might be worth asking around the local hairdressers.
Boys (far more than girls) sometimes find work helping market traders. As with the hairdressing jobs, the posts are never advertised. You have to go and ask.
Some cafes also take on young people to help out. Without a food hygiene certificate, you won't be allowed to get involved in food preparation; the work usually involves clearing tables and washing up. (Both boys and girls seem to do this type of work).
The rules which apply to the employment of school-age young people are explained here:
http://www.direct.gov.uk/Parents/FamilyIssuesA ndTheLaw/ChildrensRights/ChildrensRightsArticl es/fs/en?CONTENT_ID=4002945&chk=AaZA0p
Good luck with the job hunting!
Chris