http://www.direct.gov...itiesatwork/DG_066272
Tells you the law etc. You're not eligible for minimum wage until you're school-leaving age (16).
It will be really difficult to get a job at the moment, due to the high levels of unemployment. By law you're not allowed to work many hours, only two on a weekday or Sunday and five on a Saturday, so you're not a good prospect for most employers in the current climate. Could be a Saturday lunch waitress, but bear in mind your restricted hours are not necessarily appealing to employers.
Maybe try and increase your baby-sitting, ask the people you baby sit for if they have any close friends or family that you could also work for (as long as they know them and it is safe). Or offer to do odd jobs for family like raking leaves, cleaning out garage etc. Not terribly exciting, but you could probably earn a bit this way at this time of year. Do your parents/relatives run a business, or know anyone that does, who could give you a Saturday job? You're not eligible for minimum wage, so make sure you're not exploited (that's why I suggested friends/family). Also, got any clothes/shoes you no longer wear, but are in good condition, could eBay them (or any other random stuff that's cluttering up your room).
Good luck