Hiya, I only did my sixth form application last week so I was in a similar situation
I started at the top with my contact details.
Predicted grades/Qualifications
Achievements (in/out school)/ Hobbies/interests/work experience/volunteer work/employment and a brief statement about what skills they show - just pick the main examples so it doesn't get too wordy!
A bit more detail about your personality - really sell yourself without lying and perhaps why you want to do the course(s) and you aspiration for the future.
Hope this helps- the best thing to do is to make it concise but show yourself in the best light