Your extracurriculars essentially define who you are. You should typically show how you are a unique individual that can contribute significantly to your college's campus. Follow your interests first. So if you are a science person, join the physics club. Like to lead others/interested in politics? Check out student government or the young democrats club. Visit your guidance counselor for a program of studies/survey of activities (usually they are jointly published) that lists your school's available clubs, their advisors and meeting times.
If you start off by following your interests, you will join activities that you will go far in because it is where your passion lies. This passion will make it easier for you to shoulder responsibilities and go far in your club. Things like leadership positions, high participation, competition awards and initiative in your clubs is what is most important; membership alone cannot carry you. Be a leader in two to three clubs and join a few other clubs that you still are interested in/have friends in/want to learn about.
You should join clubs early (freshman year) so you build a relationship with club members and advisors. Active membership will get you a better chance for a more in-depth and rewarding relationship; being known will make it easier to earn officer elections.
Feel free to join clubs after freshman year though; your interests may change (will change) during HS so you can be flexible.
It would be wise to be involved in a wide array of activities. For example I ran track just during sophomore year, while being a guest writer for my school newspaper during senior year. Yet I was involved in Model UN and Key Club all four years, holding gradually higher officer positions as I grew older. This way you are not a type-A student, just focused on science or just into writing English papers. You want to stand out by being focused in a certain area, but there must be depth to your interests!
Keep this in mind as you plan out your high school activities. Your time is limited so choose wisely!
