Combined BS/MD program applicants fall into a variety of categories:
1) Students that KNOW they want to become a doctor
2) Students that are fairly sure they want to be a doctor, but aren't entirely sure
3) Students who have no idea, and are just applying as a backup.
Good news: If you're the 1st or 2nd kind of applicant, you're set to go! However, if you're the 3rd kind of applicant, you should consider spending more time looking at alternative programs. BS/MD programs are designed to ensure they are accepting the most committed students.
Before getting all your applications ready, make sure Medicine is the field you want to get into. Sure, you may be only 16 or 17, so how can anyone expect you to know what you want to do for the rest of your life? This is the tricky part.
When it comes to choosing medicine, direct BS/MD applicants are asked to make a pretty serious commitment (probably the ONLY real commitment they've ever made in their lives). Combined BS/MD programs want to see this commitment to medicine through your essays and your interviews.
Your job through your time at high school is to find out if this is what you really want to do, so that when it is time to apply, you can prove to the BS-BA/MD admissions committee why you want to pursue a career in medicine. Even if you're 100% sure already, you still need to be able to show the admissions committee that you've explored the field (through shadowing, volunteering, etc) and because of this, you know you want to be a physician.
For some percentage of applicants, you may not ever know with 100% certainty that you want to be a physician. Even during medical school, you may question your choice and your heart may waiver. But know that you're about to enter a remarkable profession where you're saving lives every day. When you start forgetting, shadowing and volunteering will enable you to see what you will be spending the majority of your life doing.
Synopsis: Make sure you are doing something you love.