Hi @Green22 -
Kudos to you for thinking ahead to this extent! In addition to the very good advice provided by others on this thread, thought I'd chime in w/ a few perspectives.
1) When should I start studying? I plan to take the MCAT in 2019 because I am aiming to start med school in the fall of 2020. I am thinking about starting as early as this semester (lightly with maybe Barron or Kaplan flash cards..nothing extreme yet)
As other posters have said, 3-4 months of MCAT studying is a pretty standard timeline. The one caveat I would make is that you may want to adjust this based on what else you have going on in your life. For some students who are balancing MCAT study with full-time study and/or work, longer study schedules can make sense, although you do have to be very systematic about it and avoid stretching it out to a point where you burn out or have trouble consolidating the knowledge. As a rule of thumb, I would be very careful about planning for anything longer than 6 months on one end, or shorter than 2.5-3 on the other end.
In the meantime, as others have said, focus on doing well in your prereqs,
and on learning the core material in a longer-term, more active way, with an eye towards building connections among the core science areas. (For example, how can physics knowledge help you understand key biochem concepts better?). Even successful cramming in prereq classes isn't really going to build the study habits or long-term knowledge that you will need.
2) When I do get into studying for it, what methods are the most useful? I like flashcards...but will they truly help me?
Use flashcards sparingly at best. They tend to foster passive knowledge. At its core, the MCAT is a test of your ability to
apply your knowledge, not regurgitate it, so you need to develop a more active knowledge. Doing frequent, realistic practice problems & careful analysis thereof is key. I'm also a fan of the idea that the best way to learn something is to teach it. Study groups can come in handy here, because each person can take certain topics and teach them to the others. Even if that's not doable, a baseline assessment for yourself is to figure out whether you can explain a certain topic clearly and correctly in your own words.
3) (if my first score isn't satisfactory) Can I take the MCAT again and up to how many times? Does taking it more than once do more harm than good?
People do retake the MCAT and get into med schools, but it's not an advisable plan. Plan to take it once and do your best, postpone if needed, and only retake if absolutely necessary. Don't let the possibility of retaking hinder you from doing your absolute best the first time.
Hope this helps, & best of luck on your journey!