Check the application guidelines for the college you're interested in applying to. Although there are general rules for any college application, each university has its own rules.
Consider community colleges. While most programs require a high school diploma, community colleges sometimes admit students regardless of their performance in high school.
Obtain a General Education Diploma. The GED is designed to be a high school equivalency test, and many colleges accept them in lieu of a traditional high school diploma. Websites such as Diplomaguide.com have lists of places where you can obtain a GED.
Take a state-administered test if you are a home-schooled student. If you live in the United States, you may be able to obtain a home-schooling diploma. Providing this diploma with your Scholastic Aptitude Test results will boost your chances of acceptance.
Create a portfolio of your experiences. Some colleges will admit students who they deem to have substantial life experience. If you've been away from school for a long time, put together proof of your accomplishments and submit it to the college.