Start with people you know - Mum, Dad, Brothers, Aunts, Uncles Friends and family friends and ask them if they know of anywhere who could offer you a job. Rarely will you see anything advertised.
If you like the idea of working in a hairdressers - phone them - or better still just pop in and ask. Also tanning shops, cafes and small independant shops sometimes need extra help on a Saturday.
Paper rounds are another possibility, as is leaflet distribution. If you are more enterprising you can also consider dog walking, car cleaning, ironing, gardening etc.