I'm one of the few (I guess) that I knew what I wanted to do ever since I was a kid. I always loved science and I wanted to do research for a living. With this in mind I went to the University and made sure I stay in the track that could potentially lead me to my goal. I was one of the lucky ones that made it and now I am doing (almost) what I always wanted.
My advice is not to panic. Being in the University is not only about the particular subject you're reading. It's more to do with the general experience and the education of the mind. It's true that you will learn a lot on the chosen subject but more importantly you will learn to use your mind, ask intelligent questions and know where to look for answers to your questions. In my opinion that's what the university is about (trust me because I do that for a living). During your years there you will come across topics that you like, topics that you hate and topics that you never knew they exist. You will get an overall picture of your chosen area and by learning to use your mind in an ordered and analytical way you will start formulating more concrete ideas about what you want to do in the future. Always try to remember that you will get your "dream job" by your "soft skills" (i.e. how you think or how versitile you are in your approach to any problem) and not (only) by your knowledge of any particular subject. Finally I would suggest to try to enjoy the experience. You're in the best years of your life and you have a unique opportunity to meet people, experience life in a protected environment (you have responsibilities only to yourself after all now, later you will have for others as well) and set the bases for your future (personal and professional) development.
Good Luck and I'm sure everything will be fine in the end. After all it's not the final destination that counts but the travel to get you there. Enjoy it as much as possible
