hey maybe try working as a cashier or help a relative with some work around the house such as washing up dishes or the car they will really Appreciate the car because it cost loads to get it washed smwhere else
sainsburys used to have the 'cookie club' 15 year old could work a few hours after school. This was about 4 years ago because my son did it - google it to see if they still do the scheme. Most Saturday jobs/retail you can do as soon as you get your NI card through (usually in the may of your 16 birthday)
Good luck!