Volunteering and working in non-clinical environments gives you real life experience that you can talk about. If they ask you about skills then you'll have loads of experience to go back onto such as teamwork, leadership, communication etc. and it will look nicer than the usual stuff most people use from school.
Check hospital websites, that was how I found and started volunteering. Took a while to get everything sorted out. There may be age restrictions so be aware of that. If any friends or relatives who are Dr's or relative seeing a Dr or if you are, you can ask them if they are willing to let you shadow them. Otherwise, you can phone up GP's/hospitals to enquire about shadowing.
It's not necessary to shadow a Dr, but it does give you good insight in how things work etc. Alternatively, the next best thing everyone does is volunteering at a care home/hospice. I believe they are usually short staffed and always love to take someone on. Its what I heard, not from personal experience. Try ringing them up, usually in healthcare setting volunteering there may be a age limit, an informal interview just to see your not a bad or weird guy, some forms to fill out etc.
It is good to see you are starting early. You should also get involed in some clubs and sports in school just to show your a wellrounded individual - not a person whos working constantly, knows how to relax and enjoy themselves.
Good luck!