I just got a job, and I'm 16 and I have no experience.
Basically:
1) Make your CV easy to read. My friend's stepmother is an employer and she GLANCES over them briefly and puts them into groups through that. Presentation is important. Try to change the generic layouts too: you wouldn't believe how many CVs look the same!
2) Be confident. The worst that can happen is they say 'No, sorry, we haven't got any vacancies'. However, only try shops that you actually like and buy things from, otherwise it might be difficult to convey enthusiasm in the interview, should one be required.
3) You don't need to put your mock grades on. I put on mine under 'Qualifications' "GCSE examinations taken and expecting the results in August.... etc". If you want something extra, just put 'passes expected in the following subjects...'
4) Make sure you have a reference, or two if you can. I put down my headteacher and the mother of someone I tutor sometimes.
And finally, shops won't be asking for people who are only interested in short term jobs when they have permanent job seekers at their disposal. Wait until Christmas if you really want a temp job.