Very briefly, here are some things that are best done during your first year of graduate school:
A piece of advice I heard from a professor once was this: young people tend to agonize a lot about decisions when they have many choices. But experienced people know that exactly what decision you make, between several similar choices, doesn't really matter much. What does matter though, is that you make a decision. This applies to picking advisors, choosing courses, and picking research projects. It is more important that you make a decision, than exactly what decision you make. The important thing is to find an advisor and get started on research early, even if it's an independent study project or a first-year project and even if it's not your ideal reseach project. I've been told by professors that many students make the mistake of not starting on research as early as they could, and this can slow them down later.
If you don't already have a professor that wants you to work with them, talk to those professors you are interested in working with, and ask them some of the questions above. Ask them or look up what classes they teach, and take some of those classes. Many professors have one or two classes in which the best few students are given the opportunity to work with the professor and eventually join their group. If you can't take any classes from some professors that you really want to work with, ask if you can do a first-year project or independent study project with them (this would probably be without funding). All professors typically like to "test-drive" each student during the first year or so, to see whether the student is motivated and mature enough as a worker to do research with them. Remember how I said a student is a huge financial commitment for a professor? So you can't blame them for wanting to see how things go for a year before committing to a student! Also, it's good to ask the few professors you're most serious about working with, whether it's okay for you to attend their group meetings. Some professors may even be offended if you ask about working with them without having taken any of their classes or regularly attended their group meetings. Many professors have weekly group meetings where the members of their group present updates on their research, or they review some papers in their field, or someone gives a presentation to their lab. Professors tend to view regular attendance in their lab meetings as a sign that you are interested in their research. But don't frivolously attend too many lab meetings; this is a waste of your time. Professors will still consider you a more worthy investment if you produce work for them, than if you just sit in their meetings all the time.
So while it seems strange that after getting into a top-level school, your success is still not guaranteed, this really is just a consequence of the fact that universities are still tied to the constraints of the real world. Most students who want to get the degree will get the degree, even though no PhD is ever easy or without bumps in the road. The keys to success are: sticking with the work even when things are uncertain or tough, learning about the process of doing research, continually gathering information about funding and the general situation of your lab and department and field, and working on something that you find some passion in, even if your work is not satisfying your idealistic dreams about what research should be like.