A reference allows your school teachers or referees to write something about you that contributes to your gaining a place in medical school. It should fit in with the rest of your application (grades and personal statement) so that it includes mentions of activities that you did not have space to mention (or wanted it to come from an authority figure).
A bad reference will have a negative impact (unless there are extenuating circumstances). A good reference will not harm your choices. There are several ways on how a reference can make a meaningful impact to your application:
Mostly this is all done automatically, so either your form tutor or teacher will write this for you as part of your application process. However if you are not a typical applicant and can choose your own referees then the below are suggestions:
Form tutors or headmaster
Most students find out that their “schools” will be writing their personal statement. Try and find out who exactly will be writing your statement, and give them your personal statement as well as trying to aim for a meeting so that they get to know you on a deeper level. If there is something important that you would like your referee to know then be organised and proactive and don’t leave it too late to tell them about the amazing volunteering that you have done for example, or the relevance of your application to a certain medical school.