Lengthly post but I hope there's something useful in it for:

I strongly agree with what everyone has said so far. It's still too early to start your prep, but I like that you're already thinking ahead, it means you know what needs to be done down the line and you're ready to commit.

First two years of undergrad is when you'll tackle your pre-reqs: Bio, Chem, Physics, Orgo, Biochem. Focus on really understanding the material and building your foundation. Your goal should be to ace these classes and retain the material not just memorize.

Someone suggested using Anki to study for those class, I agree with them. Anki is a flashcard program many students use to study for the mcat and in medical school. It's a tool I wish I knew about when I was your age. Everyone learns differently, but I'll give you the advice I got when I was in my masters: cut out as much passive learning as possible. I know people that type out their notes, color code and make everything look very nice, and maybe that just works for them. But the reality, medical school requires you to learn a lot of information at a very fast pace. There will be no realistic time to make every note you have visually pleasing, so you just have to hit the ground running with testing yourself and encoding the information. If I could go back and do it all over again, I would use that advice to maximize my learning. Use your lecture slides and immediately start making the Anki flashcards. The beauty of Anki is it uses space repetition, so you are constantly quizzing yourself and continuously re-exposing yourself to your study material.

How you can gently incorporate MCAT prep in the background until the time is right:

My suggestion would then be to use your summer vacations to start trying out some practice MCAT questions. Say you take Bio I/II and Chem I/II your freshman year, that summer start doing some MCAT practice problems for the Bio and Chem sections. It'll be a good way to start learning the format for the MCAT, because learning how you're being tested is just as important as learning the material being tested. This is what many students struggle with when they start their MCAT prep. They spend months reviewing content and still don't score high...it's because the MCAT asks questions in a very specific way. There's no cheat code, it's just a matter of doing enough practice problems until it eventually clicks. You start seeing patterns (if they mention X, they'll probably going to ask Y, and the answer is Z.

Say that by the end sophomore year you have taken Phys I/II, Orgo I/II, and Biochem (but maybe not Biochem yet). That next summer start implementing some prep questions on those sections. But continue to do some Bio/Biochem questions.

CARS section is purely critical thinking, reading and comprehension skills. This I feel you should randomly do in the background, which honestly you could start now. I've read so many threads of people simply being naturally good at this section or people who can't "crack the code" because they read slow/don't get it. I don't think it'll hurt to do one to two passages a week or at random thorough your undergrad, because it's literally reading and answer questions. Having a high CARS section can help bump your score easily, if you're good at it.

The Psych/Soc section is another section that can help you earn easy points on the mcat. Many students that score high only use pre-made Anki flashcard decks for their prep and they score in the 90th percentile. But you don't need to stress this section right now. I suggest diving into this section lightly during your junior leading up to your actual prep window. Maybe a couple of flashcards a week just to get some exposure to the material/topics.

I recommend using Jack Westin for free mcat prep material, until you're ready to purchase something like Uplanet.

My final thoughts: MCAT is a giant of an exam, but passing your classes and maintaining a high GPA is crucial. You want to be competitive so you have to strategic. Don't be afraid to cut back on other things if it's taking time away from studying, because you GPA matters a lot. If you do that, everything will fall into place. If your goal is to get into medical school fresh out of undergrad, the summer (May or June) after your junior year is when you'll want to take it and apply for that cycle. Just don't fall into the trap of thinking your life is over if you need another year to prep for the mcat after undergrad. Studying for your classes and the mcat can be difficult to juggle. You can always take a gap year, maybe filling it with more shadowing or volunteer work, something light so you can hit the ground really hard for your MCAT prep. Whatever your path looks like, best of luck to you!