Employers who employ young people need to get a child employment permit from the local authority. It is there to make sure you will be safe when you are working. You should ask at school about the permit scheme and make sure that there is one in place if you are working.
If you want to work full time, you have to be 16, but you can't be involved in serving alcohol until you are 18. At 16, you are entitled to receive the national minimum wage for those aged 16 to 18, provided that you are old enough to leave school.