Heya. I don't know company specifics, so apologies for not providing those.
You can start before you're 16 if you have a National Insurance number. Some people get them a couple of months before their birthday, but you need it for tax purposes.
Pay: obviously it is company specific, but if you are under 18 I imagine it will be between £4-4.50 an hour. This may go up after 3 months or a probation period. Again though, this is a guess.
Things to include on a CV: all your personal details (name, address, date of birth, location, age, contact details etc). A short personal objective statement i.e. what you aim to get out of a job (don't say money....say something like teamwork skills, or job satisfaction etc). Education....so include your predicted GCSE grades. Any previous work experience (did you have a 2 week work experience at school?). One or two interests to show you're a normal person (don't go into depth). Any skills you have relevant to your job (for example, "I am a good listener, so I will be able to listen to customers and identify their needs.") And a refence, so I would put down your school tutor.
I have attached my CV. Hope that helps. Let me know if you have any more questions.