Use the rejection letter from the college or university that you had applied to as your tool to develop a new plan.
Contact the college or university to find out why you received the rejection letter. Work with your parents or high school counselor to make a formal inquiry as to the reason of the rejection. Something may have been incomplete on your application that you could easily take care of.
Make an appointment with the admissions counselor at the college or university from where you received the rejection. A "person to person" conference is best, but if travel and time are issues, you can opt for a telephone conference. Just the fact that you're making all this effort, may give them reason to reconsider your admission status.
Ask if your rejection for admission into a certain undergraduate school, such as liberal arts, is due to it being full. Ask if they would reconsider your application for undergraduate studies in another field such as education or psychology. The goal is to get your feet in the door as a freshman, take all the required courses of any freshman and then transfer to the undergraduate program that you really want a year or so down the road.
Request that you're admitted on a summer probationary basis. This isn't such a bad idea, as you take a few classes in the summer before the fall semester begins, really concentrate to get the grade point levels up and then, you're in for fall.
Be realistic about the rejection letter. This is not the end of the world. Go with your second or third choices in a college or university. Many college graduates received rejection letters to go on to having the time of their lives at their second or third choice in a college or university.