Keep a look out for signs in your local shop windows asking for part-time help. Make sure that the ad makes it clear what you will be doing. Vague advertisements can spell danger. It may be worth calling in on shops where you know the staff and asking if they need anyone. Stay away from places which make you uncomfortable or nervous.
People sometimes advertise odd jobs on cards here.
If you spot a possible job this way, ring the number given and find out as much as you can about the job over the phone. If the person wants to arrange an interview, take along an adult you trust - preferably the person who normally looks after you, and if the employer offers you the job over the phone, don't rush in. Say you'd like to bring this adult along to meet them and discuss the work first. Honest employers won't be put off by you involving a responsible adult. Talk to your family or the people who look after you about anything you're considering and never go to strange addresses alone.