Your grade-point average and high school transcript are the most important elements of your college application package, and a low or mediocre GPA can mean rejection. While you don't necessary need a perfect 4.0 GPA to get into college, the better your grades, the better your chance of being accepted. College admissions officers look at your academic performance over time as well. Failing math in your freshman year is not necessarily the end of your chances at college, as long as you improve over time. A consistent pattern of low achievement over all four years in multiple subjects, though, will put you at the back of the applicant pack.
It used to be that students were encouraged to get involved in a wide variety of activities with the notion of being a "well-rounded" candidate for college admissions. While being involved in activities is still important, colleges look for quality over quantity. Joining every group at school during your senior year for the purpose of padding your college application is not going to impress the admissions committee and may even work against you. Instead, commit to the groups or teams that interest you most and seek leadership positions in those groups. Colleges are looking for candidates who show maturity and commitment. If you can't demonstrate that through your extracurricular activities, and other aspects of your application are weak, you may not receive an acceptance letter.
Although the emphasis on test scores is declining, and not all colleges require you to submit SAT or ACT scores, how you perform on those tests can affect your college application. Combined with a pattern of academic underachievement, poor test scores might indicate that you are not committed to your education or are unable to handle a rigorous academic program. However, because so many factors influence success on standardized tests, admissions officers might use test scores only as the deciding factor between you and another, equally qualified candidate.
Most colleges require some sort of essay as part of the application package, and a poor essay can prevent you from gaining admittance. Not following the instructions or answering the questions, or submitting an essay riddled with spelling and grammatical errors can reduce your chances of being accepted. The essay is another indicator of your capacity for success at the college, so if yours is poorly written, the admissions committee might decide that you're not ready for the challenge of college academics. Plagiarizing any part of the essay is also grounds for immediate rejection at most schools.
Colleges emphasize integrity, and if you demonstrate a weakness of moral character, expect to have your application rejected. Lying on the application, falsifying school records, embellishing your involvement in school activities or getting caught cheating or violating your school's code of ethics or behavior are all grounds for rejection. A college can rescind an offer of admission at any time for these offenses, so even if you've already been accepted, it's important to keep your grades up and behave properly.