My D is finishing up her first year, so I know a little, but I am sure that others who know more will chime in before the day is up!
You can take any courses you want! That is, providing that they are not filled by the time you register, and providing that you meet any pre-requisites (some upper level classes, for instance). It can be daunting for some folks who are more used to a set curriculum, but for others it is a chance to stretch and explore. Not only can you take any classes that you want, but you can also take them Pass/Fail-- any or all of them. It is not really recommended that you do that, because some majors (science, math, e.g.) may need actual grades for grad school, but Brown does not compute GPA. And, the Open Curriculum and Pass/Fail (actually called S/NC-- Satisfactory/No Credit) does not make for a bunch of slackers. It makes for a bunch of very interesting and interested people.
You do have advisers who are there for you if and when you need them. As for med schools and other graduate programs, I don't know much, but I am sure that if you want to pursue med school, you will do sciences and maths. But you will also do humanities and liberal arts, because you will want to know lots about lots of things! There are many threads on this site about Brown's Open Curriculum. I strongly suggest you read Modestmelody's explanation about the Open Curriculum, the link to which I am posting:
The Brown Curriculum and University-College Explained
This is a wonderful resource!
Enjoy!