In some ways I wish I'd know I wanted to do medicine at 13 - would have meant I could have used my Year 10 work experience for something clinical rather than mucking about in an outdoor pursuits shop and saved myself some paperwork and application letter writing later on :P
I did seperate sciences, English lit & language, maths, history, religious studies, Spanish and double engineering (which was a bit dossy, but was great fun and added some variety to my timetable) and got 11A*s - I didn't know when I chose my GCSEs that I wanted to apply for medicine, and to be honest if I could go back and change them I wouldn't. Getting the required grades in the core subjects and to do the appropriate A-levels is the important bit at GCSE in my opinion.
As has been said though, academics aren't the be all and end all, but don't get too bogged down in work experience/volunteering for your personal statement either. Get the balance right between work and having a life outside of your UCAS form otherwise you'll finish school and wonder where half of your childhood went :P