http://www.direct.gov.uk/en/YoungPeople/Workan dcareers/Yourrightsandresponsibilitiesatwork/D G_066272
This outlines what you can and can't do.
However, at the moment due to the high unemployment levels it will be hard to get any sort of job, as you have no experience (well actually for anyone). All you have going for you is that you'd be cheap (in the employment sense, not being rude!).
It's a particularly hard time at the moment. You might have to get creative. Got anything you can sell on eBay? Paper round? Gardening or Cleaning or dog walking, for relatives or people you know well who would give you a small wage (not advocating trying to get work from strangers!). Car cleaning is popular in hot weather..
The laws are there to protect you, although I understand it can be frustrating. Just try and make the most of school etc. and get ready to put your entrepreneurial spirit to good use when you're old enough and market condition improve (the employment in this country is really against you at the moment). I was a housekeeper/chambermaid in a hotel from 16 whilst doing my A-levels, which paid reasonably well, was easy and meant I only worked a bit at weekends, but had opportunities to work more in the holidays.
Try and stay positive and don't get offended or upset when you get turned down. You can always ask why to help you improve, though at the moment it's likely to be too many people and not enough jobs. The fact you actually want to work will stand you in good stead, it's just a crazy time in this country (well a lot of countries) right now