Do what you can each day to make tentative decisions on what you choose to learn. The first thing to consider is your personality. You may have taken tests in school to help you find this out. Make a list of what you know. Ask parents, friends, teachers how they would describe you.
Many high schools give interest tests.
Your interests are strongly linked to careers and may help you identify possible college majors or programs of study. Abilities and skills are also quite important. I tests will provide you with objective measurement of your verbal and math skills. Some tests even measure things like your spatial reasoning, your mechanical aptitude, and your clerical speed and accuracy. Again, these results can provide you with possible fields of study.