it depends- like many schools there's probably a process like filling out a form, giving an assembly or having an interview. Find out what the process is for your school and do it. Remember to sell yourself in ways other people want to see- i know how this sounds but especially if they have a voting system you need to please people, make sure you can handle the responsibility with your A-levels cus some schools give a lot of responsibility to the headboy/girl and be authentic and amicable and ofc enjoy it
hope this helps