You can get a part time job from the age of 13, but only for a small number of hours, this increases from your 15th birthday.  

If you do start a part time job, your employer has to have a work permit for you, you should be given a copy.

There are certain jobs you can't do, like selling alcohol, delivering milk and selling medicines.

If you start a part time job, your employer is responsible for ensuring you are kept safe.

You can find more information at Directgov - Child Employment.