Graduate work is very different from undergraduate education, in that you will not be taking courses, but rather doing research in someone's lab for 5 years. This implies a very close intellectual relationship with a small group of people (the lab members) and an important and not-always-easy-to-navigate mentoring relationship with the professor.
For that reason, a large part of the application and selection process centers on finding a good fit rather than just how smart you are or what are your grades -- we often try to take into our labs people whose interests align with what we do. We also look for people who we get along with well, where the conversation flows, etc.
The former kind of fit (topic-wise or method-wise) is one you can and should address in your research statement -- what we want to hear is not how great your grades are (that has relatively little predictive power for how you will be as a researcher) but rather all about your past research experience (even if through coursework and papers that you have submitted -- anything that involved you independently researching a topic and answering a question), and what you want to do next.
This description does not have to be very detailed but speak about your past work in ways that show us your independent thinking, your ownership of the work. Basically, we want to know that you know what you are talking about, and it is not just that you blindly carried out what someone else told you to do. Then, when talking about what you want to do in the future, again, you don't have to be specific, but just saying "I am fascinated with the brain" does not demonstrate deep thinking. So try to show us that you have thought deeply about graduate school and why you are applying, that you have thought deeply about this particular school and why it is a good fit to your interests in general (see below). You will not be held strictly to what you write -- you are allowed to change your mind later, and we know it happens frequently. You are still young.
One way to demonstrate the good fit is to mention in your statement of purpose professors that you would like to work with. By mentioning names you give us an idea of the areas you are interested in, in one short sentence. This is also very helpful to us, as we know who to ask to read your application. For this, you should look at websites of different faculty etc., to make your choice an informed one. At PNI, your application is to the whole program, not a specific lab, and you will get a chance to rotate in several labs before choosing which you want to join. So again, we won't hold you strictly to what you said in your application -- but still, it is helpful for us to know.
For the latter kind of fit (personal fit), you should treat the interview process at schools not only as "they are interviewing me" but also you are interviewing them -- that is, be attentive to the environment, your interactions with the professor (do you "click"?) and ask other students about the lab atmosphere and the advisor (you can even contact lab alumni -- they are often mentioned on lab websites and you can email them your questions).
Some of the fit (or lack thereof) you will just feel. Other things you can ask explicitly: how much time does the advisor spend with people in the lab? What is their mentoring style? Are they "hands-off"? Detail oriented? How much freedom do people in the lab have to pursue their own interests? There is no "right answer" here -- you may be looking for a very hands-on mentoring relationship and a lab where there is a project ready for you on day 1, or you might prefer to pursue your own research questions with less input from the advisor. Different people prefer different mentoring styles. It is also important to ask questions about the social atmosphere in the lab and the department -- 5 years is a long time, and science is hard, so a nourishing and supportive environment is important. If you are unsure about a lab and want more information than you feel that you got in the visit, email past lab members -- they have less at stake(as they will not be your colleagues), so they may be more open with you.
Note: Some people like to email professors they are interested in working with, to introduce themselves and make a connection. Some professors like this as they can make a short list of what applicants to seek out once all the applications are in, and they can tell some applicants that there is not a good fit, thereby saving them the effort (and money!) of applying. But other professors do not like this and may not even reply to such emails (don't take it personally!). So it is up to you whether you want to email people or not. In any case, if you do: 1) make sure your email is brief but has content. Don't just say "I looked up your site and I am really interested in your work" -- that generic message might as well have been sent by a bot to all the faculty in the department (you would be amazed how many such messages we get!), but write something that shows you are a person and you are genuinely interested in their research. 2) don't take it personally if you don't get a reply -- we are all overwhelmed with emails! It is not a sign of someone not wanting you in the lab.
Finally, if you read the above and think "hmm.. I don't have that kind of research experience," and/or "I don't actually know what I want to study at all.." then you might want to take time to acquire that experience and/or do that thinking. For the former, many go on to a full-time paid research assistant job after college, for 1-2 years. This is not only to gain knowledge and experience, but most importantly, to experience full-time research and what it entails, and make sure you really want to do that for the next 5-6 years! It is a hefty time commitment, and liking college does not mean you will like research. For the latter, you can read books, papers, etc.. try to think about what excites you. What stuff that you learned about in college really made your eyes sparkle.. what did you want to tell your roommates or friends and family about. Research is hard and you have to go through many failures for every one success, but it is all worth it if you are doing something you are very excited about. Otherwise, there are much better ways to spend your time...