A major means that you take a cluster of classes in one subject-- for example, history. Each college will specify its requirements for each major. Most colleges require students to choose a major, most often by the end of their second year at college, but sometimes earlier. Completing a major ensures that your education gives you a depth of expertise in one particular subject area, in addition to a breadth of learning in many other subjects.
Sometimes people “double major” if they like two different subjects equally much-- such as art history and math.
Not everyone completes a minor and it is very seldom required anywhere. It consists of a smaller number of classes in a subject other than your major. Let’s say you love history but have a secondary interest in political science; major in history, taking about 12 classes, and minor in political science, taking about 6 classes.
In addition, some colleges have course or distribution requirements-- for example, take three courses in lab sciences or math, three in social sciences, and three in humanities-- before graduation.
The requirements for each major, minor, course, and distribution will differ from school to school, so you will need to consult their brochures or websites.
At college you also will have an advisor who can help you sort all this out.
You may not need to worry about majors now, in high school, unless you have a very specific interest in majoring in something that not every school has (like forensic science or criminal justice), or that is available in a specific type of school you have to apply to (like an engineering or nursing school or program).
Talk to your guidance counselor about your interests, and look at a list of majors in any college you have heard of. That should begin to give you an idea of the types of major that are available.