You can work but only approx 2 hours during the week and about 4 to 6 hours on a weekend. I believe you can work approximately a total of 10 hours a week. During school holidays you will be allowed to work on a full time basis.Try places like fast food restaurants who may need Saturday help. I would contact the local authority to see if they can give you the work permit paperwork that you need to get the employer to fill in. That takes the onus off the employer to have to get the paperwork. There are certain limitations to the type of work you can do, you cannot handle any dangerous equipment and the employer should complete a risk assesment to assess that the workplace is suitable for you to be in.
I have employed 14 year olds in the past in my Subway stores. I have found that although they naturally need more supervision, they were quick to learn and very reliable. One of my youngsters stayed with me on a part time basis until she left to go to university and I would always consider applications from young workers. Best of luck, have a chat with your school who may be able to advise you further and ensure that you know your rights, some employers will not hesitate to exploit young people!