I"m 14, almost 15 and i work at a small cafe down the street. If you want something simple then go for baby sitting for family and friends of you can make fliers and do all that. But if your talking something more like working in shops or whatever you just have to get out there. Ask your parents to help you out and look as well, your of legal age to work so you just have to find someone willing to take in a first time worker.
Do what adults overcharge for at a lower cost. Perfect example is yard work if you're a boy, house sitting when people go on vacation, e.g. watering all outdoor plants every day, clean out cellars and garages, help move furniture in the house. Just a few things I would like and pay $10.00 under the table for this help.