What I would do is- phone a shop and ask to do a week's work experience (perhaps get your teachers to help you out)...even if you're not getting paid, this will give you really valuable experience and something to put on your CV. There's also a possibility that the company could keep you on afterwards (This happened to my friend with Dorothy Perkins).
Getting a job once you have some experience is easy, the only people who complain about not being able to find work are those who haven't done anything before. I've worked most of my teenage years and I've never ever had to wait around for a job- when I've wanted to switch jobs, I just find another before I leave my current workspace. I've had jobs in a bakery, a cinema and a supermarket and I'm a TEFL teacher at a language centre on Thursdays...
Once you have some work experience, it's all downhill. Write a really good CV and be persistent. Employers I would recommend are Primark (they hire anyone and promote anyone...I have two 18 year old friends who are supervisors and on £7.50 an hour) or any supermarkets as the discounts will help on your gap year and pay tends to be a lot better.
I'd start looking once January exams are out of the way and perhaps start off as a saturday worker- most places are flexible with hours and you should be able to get a decent contract when you want to be a bit more full time
(Oh and I would recommend doing a TEFL course if you want something different, I've done a 3 day intensive course and got my job at a school REALLY quickly. The company TEFL-UK emails me every other day with new, well paid positions and quite short contracts if you just want a break from mundane till work. I'm still in year 13 and going to uni so I won't be doing any jobs abroad any time soon, but they emailed me the other day asking me to apply for a teaching job in Barcelona, £300 p/w and discounted accommodation! LOVE IT, can't wait to finish uni so I can start applying for these jobs!)