General guideline: Focus on getting as close to a 4.0 as you can during your first 2 semesters in college. During your spring semester, look for a lab for summer research, either at your university (if you want to take a few summer classes) or as part of a summer research program elsewhere (eg: SURF). Focus on labs that use molecular biology techniques to study basic cellular biology problems or human disease processes, which includes the majority of NIH funded labs. You can decide what interests you the most (neurobiology, cancer, basic processes, immunology, and so on), but you should contact multiple professors, including finding them in person if they do not respond to your e-mails (they are busy and get many emails for student researchers). You need to be willing to put in serious time (20+ hours/week, potentially for 3 years). Focus on professors that have more than 1 graduate student and post-doc and are involved in publishing more than 1 research article/year.
Once you get initial summer experience (and initial training), you can decide if research suits you or not. If it does, prepare to spend the rest of your undergraduate years conducting research part-time (ideally 20-30+ hours/week) to be competitive (3 years research) for MD/PhD programs. If the initial lab does not suit you, it will be much easier to find a new lab because you have some skills. Always focus on your GPA/tests first (eg: to put in serious lab hours and keep your GPA high, you will need to be very intelligent and work hard), because you should keep your GPA above 3.8 or so. Your junior year, you will prepare for and take the MCAT. This guideline is if you plan to matriculate immediately following a 4 year undergraduate program.
There are many variations to this, but if you follow it you are likely to do well in the application process. Your major does not matter, but it should give you a strong scientific background. If you enjoy research (not just tolerate, but enjoy research- eg: you stick around in lab even after the work is done) and are also suited to medical school (shadow physicians, have an innate desire/need to practice medicine), and are willing to spend 7-9 years in an MD/PhD program even if you will never do research after that (but initially want to spend the majority of your life conducting research), then you are suited to an MD/PhD program.