Highly selective colleges like to see applicants who have a focus in one or two areas. As you go through high school you need to think about what activities really excite you. Which ones are you the most motivated to do well in? What is your passion?
You want to focus on those activities and related activities so that you develop that passion. Can you combine your passion with volunteering in a related field or even a related job? That will strengthen your application even more.
Don’t just join organizations because you think it will make you more well rounded. Most selective colleges don’t want well rounded students. The shot gun approach to activities, where you participate in lots of unrelated activities, does not help you with admissions to selective colleges.
Does that mean you can not be involved in different unrelated activities? No. But try to make sure that your real passion is obvious to all who know you. That passion can carry you a long way toward admission to a selective college.