Bookgirl is WRONG. You can take college classes, you don't need to take SATs. If you are taking an English or Math class, you will have to take a placement test.
Yes, you can take classes online or at your local community college while still in high school. Most states offer these classes to high schoolers tuition free, but you will have to purchase books. You would be what is called a "Dual Enrollment" student. You need to go to the college's website, and look for information on dual enrollment. You will need to get permission from your current high school.
My daughter took an economics class at college when she was in 10th grade. She did quite well. In the fall my homeschooled 12th grade daughter and 11th grade daughter will both be taking classes at our local community college.
You have to have a ged or diploma first and then take an entrance exam (either specific to the college or generic like the SAT). There are some distance learning you could do for college credit (especially involving history classes) but they may not be useful to whatever degree you decide to pursue.