It's hard for me to offer any advice that deviates from the predictable:
Do well in school, have a solid GPA (science as well as non-science classes)
Good MCAT score
Strong letters of recommendation (from practicing MDs is nice if they know you well, bonus if they're well-regarded)
Decent range of extra-curriculars
Volunteering and research is a plus
Of course, when starting undergrad, I'd definitely advise you to make sure you don't become too focused and narrow-minded with regards to medicine. Make sure to balance your life, take time to make good friends, stay healthy, etc.