Science and math classes will help you for the undergraduate classes you will need to take to prepare for medical school. Students pursuing medical school usually major in biology or chemistry, though if you don't you may do a "post-bacc" program.
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.
MSN, RN, WCC, OMS, ONC- Medical Surgical Clinical Nurse Educator
Greater Boston Area
Also, consider taking latin. Many of the names of body parts and conditions are derived from Latin, so it makes everything make a little more sense and is easier to figure out :)