Have a read of this.
http://www.connexions-direct.com/index.cfm?cat alogueContentID=120&pid=75
Getting a job when you're under 16 is not as easy as it was when I was young. Many shops won't employ you on the tills. Some because they sell alcohol and you have to be 18 to sell alcohol, others I don't know - I used to work for Warner Brothers and they didn't employ anyone under the age of 18.
The easiest job to get is probably a paper round, but you could also try other places, including the ones you're interested in. Also try local shops, sometimes they want younger people for shelf restocking. Not very exciting, but it may lead to other things.