How can I find a good open source project to join?
I'm currently a student in high school, and my passion is in programming and computer science. I love it, and I think I'm fairly good at it (at least compared to the average person of my age). I've taken a number of AP courses, especially in mathematics, and I also have taken AP Computer Science; though I've been reading books on computer science and programming since a fairly young age.
I was wondering what an appropriate way would be to go about extra-curricular activities involving programming (or computer science). I'm looking to go to a decent college, and I know many of them look for people who pursue their interests, and I want to find a way to show that I really am pursuing my interests.
I've heard that open source projects are a good way, but how can I prove that I actually contribute to a project? And how would I go about finding an appropriate project to join that will allow me to join? Are there any alternatives to open-source projects?