Narrow down your interests--those which could become future careers. Ask your school counselor for written information, such as brochures, about the majors you're thinking about and the colleges or universities you're interested in. Read through each and hold on to the ones which seem like they might meet your interests and future educational needs best.
Don't pigeon-hole yourself in high school. Explore your interests with your school counselor and, on your own, study brochures and catalogs and take your time. When you do register for college classes, don't declare a major; sign up for your "general education" courses, which you have to take first.
Explore introductory classes in the fields which interest you. Use your experience in those classes to decide which career(s) still interest you the most.
Re-read your college catalog for the descriptions of each major you're interested in. Look for what the classes will prepare you for and what classes are required. Continue to take introductory classes in the major(s) you're interested in. Begin to narrow your choices down at this point.
Don't try to take a class in a major which does not interest you--don't waste your money or time. Keep in mind which classes have held your interest. Before declaring a major, try to take additional classes to ensure you are interested in that field.
Have a heart-to-heart talk with your parents if you're getting pressure from them. If they are indicating you "should" major in something which doesn't interest you, let them know that the major they want you to take isn't something you want to pursue and that it would be a waste of their money for you to pursue the major they are set on.
Change your major by filling out the required paperwork in the department or college which houses the major. Pay attention to deadlines and other required tasks, such as writing essays to be submitted with your paperwork. Don't forget to drop your current major in your current department or college.
Make note of the required classes in your new major, after dropping an old major and declaring a new major. Get a copy of your unofficial transcript and highlight which classes you've already taken. Look through the college catalog and highlight which classes you still need to take and make a plan to get those classes within the next few semesters or quarters.
Take summer classes if you have several credits remaining in your new major. You may save money; summer session classes usually cost less and you will receive more personalized attention from your instructor or professor. You will also open up some time in your upcoming fall and spring semesters or quarters in which you can apply for an internship in your major.