My advice to everyone is to first and foremost learn to program. Programming is the most useful skill across engineering (and science); it helps in every branch of engineering so do start there. Next, if you are interested in robotics, read up a few books, starting perhaps with "The Robotics Primer" I wrote back in 2007, published by MIT Press. Reading a bit about robotics will help you to decide if you are more interested in programming robots (their brains, that's what I work on), or on building robots (their bodies). Programming is studied in computer science, while building is studied in mechanical engineering. You could also study electrical engineering and from there learn about sensors and go into building or programming. So those are the college majors to pursue if you know you want to get into robotics. Well before college, I strongly recommend getting into a local or more global robotics competition, there are many around you could try. I especially recommend Botball (http://www.botball.org) which covers elementary, middle, and high school; but FIRST is also good (focus on the LEGO league if you want to program!), and really any option you find will give you experience. These days there are increasingly more affordable robot kits you might be able to purchase and learn from at home, perhaps with friends on a team. Depending on where you live, look for local summer and after-school robotics and programming classes and activities, as well as any your school may offer. Also look at local universities and go for a tour of as many labs in engineering as you can. Once you are a senior in high school, if you have programming experience, you may be able to talk a local university professor into taking you into the lab as a volunteer (don't expect to be paid, invest in your future :), and that is also great experience. Finally, just so you know, I never did any of the above, I only decided to go into robotics in my senior year in college: so it's never too late! In addition, I only got involved in robots for helping people in the last 15 years, and then invented a new field of socially assistive robotics; before that I worked on robot teams. So again, it's never too late to figure out your dream career and pursue it, but it's good to think about it and plan for it early.
I love my work and get a lot of questions about it. To even better answer your questions about how I got to where I am, I recommend looking at this web page, which I put together especially for girls interested in engineering:
On that page there are links to interviews, Q&A, and videos answering exactly the question you asked.
Remember, you get to invent the future of robotics, so go for it, have fun, and do something worthwhile that makes the world a better place!