They also look at SAT scores, extracurriculars, admissions essays, etc.
Don't rush into it. If you don't get into the college of your dreams right out of high school it's worth waiting for. Most kids don't know what they want to do at that age anyway and they're not mature enough. If you wait for several more years until you're more mature you'll do better in classes and you're much more likely to graduate.
A lot of colleges have a policy where, if you've been out of high school for 3+ years, they don't really weigh heavily on your high school grades or SAT scores for admission. At that point it's more about professional work experience, community involvement, community college grades, etc. You're considered an adult at that point so you're not really competing with other people right out of high school.
The key is you want to get that in-person meeting with an admissions counselor and show them that you're mature, responsible, enthusiastic and focused enough to handle it. Don't go in there telling them you feel like a miserable failure and have no idea what you want to do with your life.