If you have a reason for a low GPA such as health problems through high school, mention this in your admissions essay. Also use the essay as an opportunity to show the school you're serious about your intended major. Write about what you plan to do with your degree. Also write about steps you plan on taking to get better grades in college. Make sure to use perfect grammar and spelling in the essay. Get at least two people to read and critique the essay. Choose people who have excellent grammar and writing skills like a teacher.
If you're entering your senior year of high school and have an extremely low GPA, you probably don't have time to raise it by much. While you should still try to do well in classes because some schools look at the trend of your grades, you should also focus on studying for standardized tests. Most colleges require the SAT or ACT test for admissions. You can take these tests more than once. A high score on a standardized test can sometimes outweigh a low GPA by showing the school you're capable of college-level work.
Sometimes the GPA requirement for early admissions is lower, but usually you must agree to attend the school if you are accepted. Some schools also accept students through special programs. The programs let you attend the school with a lower GPA, but have stricter requirements than those for general admissions. Other programs accept students with lower GPAs under academic probation, meaning if your grades aren't passing or average you can be expelled from the school.
If you have a low GPA, consider going to a community college for a few semesters. Make sure to get excellent grades while at the college to prove yourself before applying to a more competitive school. Most community colleges accept anyone with a high school degree, so admission is not competitive and often doesn't depend on your GPA. The more competitive schools will overlook a low high school GPA if your community college GPA is high enough. Transferring credits from the community college to a four-year school can be a challenge, so make sure the courses you're taking transfer by talking to an academic adviser.