<p>Americankid has it down
make sure you are sincere in your passion. This may seem obvious, but I know people like this: don't just choose a random "passion" that you are sort of interested in and pretend that it's more than that. You should find something about which you are naturally passionate and do as much as you can in that area at a high level. It is much better to have a few focussed & great ECs to which to devote a significant amount of time than a laundry list of random ECs that you don't really care about. Participating just at your high school level won't usually be enough. Summer programs that you simply sign up for aren't either. I found it helpful to sign up for a bunch of clubs as a freshman, and then narrow them down as a sophomore and junior to the ones I sincerely enjoyed. In senior year I made sure I had leadership positions in them. I now do three clubs that I am very involved in. They are among three-four that are somewhat related, as well as a couple other low key clubs that I may or may not have ended up including on my app. The most important ECs are outside of school though. Summers are a great time to explore your interests. Sign up for programs in Fr. year to build your knowledge and resume or even just to see if an interest is your passion & do research to see how you can rise to a high level in your area of passion. For scientists, there is the intel competition. For debaters, dartmouth itself has a high level program. You should also reach the highest level you can in competition. Politicians of the future should look to participate in the page program or girls/boys nation. If you can get an internship in any field, that can be a great opportunity, but go above and beyond. Don't just be great with the copy machine, but take full advantage of your opportunity to be with professionals in a field that interests you. learn from them, get involved, learn from anything you can while you are there. Actors should look for the most advanced theaters & theater programs that they can participate in. Sports usually only mean something if you are recruited, but if you are, they can be very helpful. Decide by the beginning of soph yr if you are interested in playing in college at either the D1 or D3 level. If you are go for it. If not, there is certainly no harm in participating if you enjoy it, but don't think it will help you significantly for college.
EC-wise, I strongly advise you not to be too focused on college. If you are truly passionate, what I just said will come naturally if you know the opportunities available. If you are not and if your motivation to intern or participate at any level was getting into college, the adcoms will see right through that. Also, mostly if your family has a high income, interning or working for your daddy, if your daddy is someone from whom you could potentially intern does not mean much either and will most likely make it look like you have had every opportunity handed to you, regardless of whether or not that is the case. </p>
<p>That said, the most important thing is grades. No, one B won't kill you, but the closer you are to straight A's & A+s, the better. Also, figure out the course load of those who have gone to Ivies or other high caliber colleges from your school. If they take 8-10 per year, you should too. If they take 2 per year starting in junior year, your school probably doesn't offer many APs and colleges will know that. In that case, take as many as they let you. Colleges love students who challenge themselves in their given schools, so take as many APs & honors classes as you can manage, without spreading yourself too thin. I've heard a few colleges say in info sessions that a B+ in an AP is better than an A in a regular class. Within your classes, especially those of particular interest, get into the habit of doing extra work and going above and beyond what is expected. When it comes time for you to choose teachers to write your letters of recommendation, your teachers will note this. Furthermore, learn for the sake of learning, not for the sake of getting a good grade. Good grades will come with this. Just like you shouldn't do your ECs because you want something to add to your application, you shouldn't study for the purpose of getting into a college. While this certainly is motivation for most people applying to highly ranked schools, it should not be your primary motivation. More often than not, colleges will also see through that and if your teachers recognize that you love to learn and genuinely enjoy doing so, they will reflect that in your recs. Somewhat ironically, learning for the sake of learning rather than for college will end up helping in the college admissions process more than the latter.
I hope this helps you out & sorry that it is so long! best of luck!</p>