I remember back to a conversation my sixteen-year-old self had with a summer job coworker. He'd asked what I wanted to study at university and I said "science...maybe medicine in the future". "You're not going to get into medical school" was his response. He went on to explain that he'd been rejected from medical schools and believed that the only people who got in were those who were 100% committed to medicine. "You said 'maybe'. The fact that you have any doubt about doing medicine means you're not going to end up doing it."
Well let me tell you, everyone has doubts about doing medicine -- including people who are already doing it. There is no way you can know in high school, undergrad, or even medical school whether medicine's where you're "supposed" to end up. Now in my eighth year of post-secondary education I'm still not sure that I wouldn't be just as fulfilled had I taken the engineering or grad school route. The point is, you probably won't know in high school whether you really want to go to medical school. If it's something you're in any way considering though, here are some things (from my own experience) that you might want to consider:
High School Biology, Chemistry, and Physics -- You don't have to be a science person to get into medical school (some of the best physicians I know were creative writing or music majors), but what you do need are science prerequisites (up to third-year biochemistry) and to know enough physics to get through that section of the MCAT (high school physics will suffice). So when it comes to course selection, make sure you sign up for all of the sciences! If you're missing any (confession: I didn't take any biology in high school), no worries -- universities offer high-school equivalent catch-up courses (I had to take Biology 11 & 12 and Chemistry 12 in my first year of university).
Grades -- Medical schools will not ask for your high school transcript! Even universities will only look at your Grade 12 marks and a few of your Grade 11 marks. What does this mean? Your marks before Grade 11 don't matter one bit and once you've gotten into university, your high school successes or failures will disappear into oblivion. If only I could go back and tell my Grade 10 self to stop worrying about every half mark I lost and multiple choice I'd guessed on. That's not to say you shouldn't work hard in high school -- it's an important time to learn how to study and getting good grades can be pretty lucrative (see the next point).
Scholarships -- While not directly related to getting into medical school, they do help to enhance your resume (and give you something to put in the "awards" section on your medical school application) and can be a source of easy money. So definitely apply for as many scholarships as you can!
High School Extracurriculars -- A good 50% of your medical school application will focus on "non-academic experience". This includes everything from volunteering to clubs to sports to music -- and the earlier you start involving yourself in these things, the easier it'll be for you to showcase yourself as the well-rounded medical applicant that you are. Get involved in as many things as you can while in high school. As a quieter person, stepping up to run a club or lead school tours pushed me out of my comfort zone and improved my confidence. It's easier to get involved in extracurriculars at the high school level than in university, so sign up first and worry about making time for it later (it's easier to join a group at the beginning of the year than join in mid-way when you realize you can in fact spare the time).
Volunteering -- When I graduated from high school, I lost the clubs and school-related involvement that had so nicely padded the "extracurriculars" section of my resume. All that remained was community volunteer work. High school is great time to get involved in volunteering, not just in your school but in the community. Hopefully you'll find a cause that inspires you to remain active in it even after you graduate.
Part-Time and Summer Jobs -- The summer I was fourteen I landed a job at the only place that would hire me, an ostrich farm. Incidentally this factoid did come up in one of my residency interviews this past year -- and either made me a tad more memorable or painted me as a talks-about-ostriches-in-her-job-interviews weirdo. Medical school applications do question about "employment history" and while some might argue that my movie theatre, retail, and library page jobs hold no relevance to medical school, I'd like to think that having them on my application demonstrated industriousness and good time management. I'd suggest getting a job in high school, as I know I've learned so much through the various jobs I've held -- not to mention the money I've saved to put towards tuition.
Healthcare Related Volunteering/Work -- When I was in high school I thought that it'd be a good idea to get involved in some healthcare related volunteering, in part to learn a little about the field and see if it felt like an area I'd want to work in, but mostly because I thought it would demonstrate that I was interested in medicine. And so I spent Friday afternoons reading a book at the information desk in the hospital...which was the most unfulfilling volunteer position I've ever held. Nowadays when I walk past young volunteers in the hospital halls, eagerly awaiting their chance to redirect a lost visitor, I want to tell them to turn in their volunteer vests and go involve themselves in something they really care about. Healthcare related volunteering is not the ticket to medical school. Finding something you genuinely are interested in and committing yourself to it goes a lot further.
So that's my list, written retrospectively to answer the "if I could go back and talk to myself in high school, what do I wish that I'd known about applying to medical school" question. What if you're at that stage now and are not doing any of these things? Don't you worry one bit. Honestly the medical school admissions committees don't care about what extracurriculars you did in high school -- unless it's something that you're still doing in university, you won't even be listing it on your application. And as I said, no medical school will ever see your high school grades either. I'll reiterate that if you're in high school, medical school doesn't even have to be on the radar yet. The things I've listed really don't pertain exclusively to medical school. They're just some things you may want to start thinking about in high school to help you to explore your interests and develop your resume.
If you're in high school and have any questions about university or medical school, please don't hesitate to ask.
And if you're past high school and have anything to add, please do share your advice!