Is this just for the school holidays or permanent? If it's just for the hols there are a zillion things you could do....waitressing, shelf stacking, shop sales assistant, babysitting, delivering lealets, dogwalking, cleaning etc.
If you're saying you've left school I assume you are waiting for your GCSE results in August? Your results could make a big difference to the jobs you apply for. I assume you must have had at least one careers interview in Year 11?
A good start would be to make an appointment with your local Connexions advisor at your local job centre.
Good luck.