Getting involved is the main thing you need to focus on after getting good grades in challenging classes. You don't need to do one of every kind of activity, but you do need to find something you love to do and stick to it. Here are some ideas:
11 things to do in high school to prepare for college
12 ways to get involved in high school
Leadership qualities colleges look for
Tutoring techniques for high schoolers
Volunteering:
Volunteer opportunities at your high school
Volunteer activities for high school students
6 more ways to volunteer in high school
Further reading: