If you're still in high school, start developing your interest in physics early. Take extra physics classes, for instance, and join or start a physics club with like-minded individuals. Participate in science fairs. Apply for science camps in the summer to continue to develop your interest. Read as many books as you can about physics, including biographies about physicists. It can help to find a role model. If there are physicists in your community, try contacting them to see if they'll help you in your pursuit. Many may be willing to have you shadow them for a period of time. Don't forget to invest time in math classes, as well, as math is essential to physics. Make sure you are well-rounded, though. To do well on college entrance exams, it helps to be proficient in as many subjects as possible. To get into a good school, you must do well on your entrance exams. In the United States, that means doing well on either the SAT or the ACT. Since you're applying for a science program, many schools may want the ACT over the SAT because the ACT has a science section where the SAT doesn't. However, it really depends on the school. To do well on these exams, you'll need to prep ahead of time. Your school may offer prep courses, but you can also purchase study guides that have practice tests. Taking practice tests gives you an idea of what the actual exam will be like, so you can go into the test with less anxiety. Look at universities with physics and science programs, or that are all-around good universities. Not only the top schools have programs in physics. Find a school which is a good fit for you. You can look into state schools with physics programs. Though not necessary, it can help to know whether you want to go into theoretical or experimental physics, though it's not a requirement.

Summary:
Focus on physics in high school, if you can. Take your entrance exams. Find the right undergraduate program.