Without knowing anything about your program, I can tell you that you will probably spend your first year rotating through different labs before you find a "home" to do your thesis in. Why don't you just ask them? You're in, so you have every right!
Oh, and, calm down They would not have accepted you if they didn't think you could handle it. The admissions committee has been doing this for years and they know what they are looking for. Did you ever get to a class the first day, for example, and when you look at the syllabus you think it is overwhelming? But then, after you've been in that class, you realize that when you take it one day at a time you can handle it. When you looked at that syllabus you were looking at the ENTIRE class, not at each individual day.
Looking at a PhD program can feel like you are looking at that syllabus. You read about rotating/ qual tests/ dissertations... it gets overwhelming. Keep in mind that everything you are looking at is spread out over 5-6 years. I'm not saying it's not a lot of work and it's easy, but you aren't expected to do everything right away. It's a process that many have gone through, and you will be just fine, like most of them were.
YOU GOT IN. CONGRATULATIONS. Now, take it one day at a time, work with your advisor, and enjoy the fact that you got where you wanted to be.