There are a lot of ways you can approach the MCAT. Studying over the summer and taking it in August/Sept. of junior year isn't a bad idea, just make sure you've taken all the classes that will be on it (physics, gen. chem, O-chem, genetics, human phys, and cell bio are the biggest ones). Another thing many people do is take some of these classes like genetics and human phys fall of junior year and then take the test early the following spring. That way you don't have to study those subjects much since it should be fresh in your mind. Either way, the sooner you get your apps submitted the better your chances typically are. Get your primaries in by the end of June (the application typically opens around the 4th or 5th), and try and complete secondaries within a week of receiving them. People will post the essay questions on this site's application threads, so you can actually write the essays before you even get the secondary and just submit it immediately.
Another thing to keep in mind with boards that Kirby didn't mention, Step 1 is sometimes the only step residencies will see depending on how early you apply to residencies and how late you take Step 2. I've heard of people dominating Step 1 and intentionally taking Step 2 late so residencies will only see their first score. I've also heard of people who did poorly or average on Step 1 take Step 2 early in hopes of scoring really high and increasing their chances of getting into more competitive specialties. This is why Step 1 is always talked about and seen as so important, because every residency sees every applicant's Step 1 score while some applicants will not yet have a Step 2 score. Step 3 is actually the most exciting imo because from what I understand you are not considered fully licensed or paid as a physician until you pass Step 3. Once you finish that you start getting paid and additionally can start moonlighting on the side to make extra money. So even after you graduate med school, you still aren't really considered a full on physician by a lot of medical professionals until you complete all your boards.