Several ways including
- Looking in the job centres
- Looking in magazines/newspapers/online (<-----be careful with online vacancies)
- Ask friends and family if they know of any vacancies
- Visit your local Connexions centre or visit their website if you are between 13-19
- Look for vacancies on boards in supermarkets
- Advertise yourself (do not give personal details unless necessary, and if you do, tell a parent) in shop windows
- Help out around the house (chores)
Try
http://x.vu/jobsearch as well, many jobs available.