You really don't need to worry about being "undecided" when you apply to Hopkins, or for that matter, any other college. While many freshmen may plan to study certain subjects, at Hopkins you actually cannot declare a major until the end of the spring in the College of Arts and Sciences. Freshmen are considered "pre-major" regardless of their intentions to study Public Health, French, Neuroscience, History, or whatever else they want.
My advisor was helpful, certainly, but I found that it really came down to classes. I really enjoyed certain classes, while others that I expected to like drove me crazy. First semester's covered grades made it easy to explore classes across different disciplines, and that definitely helped me to rule out a few possible majors. Just by process of elimination, I knew that I wanted to study something in the social sciences--I'm terrible at math, but the prospect of a pure Humanities degree didn't appeal either. Throughout freshman year I came to understand more about the social sciences at Hopkins, and the Public Health program really stood out to me as one that I would both enjoy and succeed at.
Most of my decision to major in public health came down to things like that. What am I good at? What am i interested in? And what is the overlap between those two things? It might take a while to really narrow this down, but two full years is a very long time, and will involve a lot of classes, jobs, and general growing up. By the end of sophomore year I promise you'll know what you want to study.
Saying that though, there are a lot of resources for students who are undecided, as well as students who have declared. Freshmen have an academic advisor assigned alphabetically, who you meet with several times a year. The Career Center can offer advice about internships and jobs that may sway your interests academically, as well. Professors are more than willing to speak about their subject matter, and research opportunities on campus may help you decide which major to pursue. Hopkins doesn't really offer "undecided" and "decided" academic resources, which means that there is a great deal available to you as an undecided freshman or sophomore.
Here is the website for academic advising
http://www.jhu.edu/advising/and freshman advising
http://web.jhu.edu/freshman_advisingLet me know if you have any other questions, I hope this helped!