My ds had very little to put on a cv so he wrote a letter talking about his interests, his good reports from school, his attendance awards and he attached a reference from the person who took him on for work experience.
He had also done some volunteering a couple of years ago so he had a certificate for that and another reference. He put that he had enjoyed trying different sports (not sporty at all but willing to try different things). Any new experience, even a school trip that inspired them, is worth mentioning.
It was quite a short letter 
But he was only going for a job as a washer upper and they took him on for a trial. He's been there a whole year now, has the very important title of 'kitchen assistant' and, most importantly, will actually have something genuine to put on his cv now.
What they really want to know is; are they going to be reliable, are they going to be presentable, are they going to be well mannered, are they going to be flexible, are they going to be punctual, etc. If you can get that across in a letter or cv it will really help ime.