Admission officers know that your grades only tell part of the story of who you are. For example, Harvard University looks for students who are very strong academically, but they also want students who have contributed in many different ways to their schools and communities. If your grades aren't strong, your talent for community action, politics or sports can demonstrate that you have strong potential for success outside of the classroom.
Some people will tell you that you need as many activities as possible. That's not true. According to Harvard's admission office, students who demonstrate excellence in one activity will be just as likely to be accepted as those who have excellent grades or lots of activities. If you have a few years, pick one or two activities and dedicate yourself to them completely. However, if you're a senior and don't have any activities yet, you'd be better off joining three or four activities right away.
Anything counts, even jobs outside of school. The dean of admissions at Kenyon College told the New York Times that one recent applicant had no activities, but worked 25 hours a week at the family gas station. That kind of dedication certainly counts. For most people, however, it's best to pick something you love: sports, music, government or community service. If you know what you want to study, try an activity that relates to it. The important thing is that you show your dedication and accomplishment, which translate to passion and ambition on your college application.
Not everyone can be editor of the school paper or president of their class, and colleges know that. However, you should try to show that you contributed in a significant way. You may not be editor, but if you were a staff writer for three years and wrote 50 articles, your admission officer will be impressed. You might not be class president, but if you organized students and led campaigns, you are a successful school politician.