It's good you're thinking about this, because CS is one of the few majors for which you MUST learn the bulk of your knowledge autonomously. If you're really just starting out, you'll want to become familiar with command line interfaces and an object oriented language like Java. If you decide to learn Java you must download the Java JDK (java.sun.com), which includes the compiler, archiver, debugger and runtime environment. If you're lost, tell me what OS you're running and I'll help you set everything up and get started with programming.
As for books, you should buy an 'Introduction to Java' type book (should you decide to learn Java) and work off of that until you feel like you know a bit more about languages and computers in general. Additionally, you might want to read up on computer architecture (Hennessey), because this will come up in your major. And it's just good to know. Eventually your main sources of information will be tech forums and articles, but I suggest you look at those books and get comfortable with a language before anything else. There is a lot of information out there, and it can get overwhelming. If you give yourself a strong and focused foundation, you'll be able to take in new information much more easily later on. Goalsetting.
The last thing I would suggest is to think about what you'd like to do with computer science. Do you want to take a graphical approach to programming? Do you like video games? Robotics? Artificial intelligence? Simulating? Network security? Hardware design? Cryptography? Search engines? Language processing? You should always be reading about those areas that interest you.
Again, if you need any help setting up or have any questions, just ask.