You can work if you are 13 or over, but there are big restrictions on the hours that you can work. This is mainly to ensure that your work does not interfere with your education. I know education does not seem important at 14, but one day you will find out just how important it is.
Everything Grunty said, but there are also some restrictions on the type of goods you're allowed to sell at that age. For instance, you can't sell tobacco or alcohol, and you wouldn't be allowed to work in a bookmakers or a sex shop, either. There are also restrictions not only on how many hours you're allowed to work, but on when you're allowed to work them.