Do you know what you want to do? Do you even have a CV?? LOL
Your best bet is to scan the job pages on job day in your local rag, they usually have a load of jobs you can apply for. At your age it's more than likely going to be a case of giving them a call and they will guide you from there.
Your other option is shop work, probably more suitable for you working around your school time. Dress smart (but not OTT like a suit or anything!), take a trip into town and go and ask for the Manager, tell him.her your are looking for some Saturday and/or evening work and leave your contact details with them. Also keep your eyes peeled for the shops that might have a sign in the window looking for workers.