Definitely agree with CWF that math and science courses are the most important.
In high school, make sure you do well in algebra, geometry, and calculus for math and chemistry and biology for science.
In college, typically the following courses are recommended before taking the MCATs and applying for medical school: General chemistry with lab (one year), Biology with lab (one year), Organic chemistry with lab (one year), General physics with lab (one year), English (one year). Over 50 medical schools also require 1-2 semesters of mathematics (college math, calculus, and/or statistics). 15 medical schools require a semester of biochemistry, and more will likely require this in the future. 17 medical schools require MORE than one year of biology, including many of the Texas and California state medical schools. A few medical schools have requirements such as psychology or sociology.