Some students who aren't sure what they want to study in college worry that colleges might hold that uncertainty against them. They wonder if applying as an "undecided/undeclared" major makes them less appealing than an applicant who's declaring what she wants to study.
Thankfully, as long as you're applying to the right colleges, you won't have to worry.
Colleges that offer the undecided/undeclared option are perfectly OK with students choosing it. It's a college's way of telling you that they don't expect every seventeen year-old to know what you want to do with your life. They'll probably have you take general education requirements, the classes that everyone has to take regardless of their major, so you can try different things. And as long as you select a major by the end of your sophomore year, you'll be fine.
Not all colleges offer the undecided option. Lots of schools see their mission as one to help you reach a career that you've identified (Northeastern, Drexel and Cal Poly San Luis Obispo are good examples of colleges that do this). When schools like those ask you what you want to study, you need to have an answer.
But for colleges that offer an "undecided" option, be confident that you're uncertainty is OK. If they ask you to describe your academic interests, tell them what you're considering studying and why you'd like to keep your options open. You should be thinking about those things, but the right colleges won't hold it against you if you don't have an answer yet.